To explore the cognitive function of patients with bipolar disorder Ⅰ depressive episodes and major depressive disorder （MDD）， as well as its relationship with thyroid function and clinical features. Methods From June 2021 to August 2022， 45 patients with bipolar disorder Ⅰ depressive episodes who visited the outpatient and inpatient departments of Wuhan Mental Health Centre were selected as the bipolar disorder depressive episode group， and 45 patients with MDD were selected as the MDD group. During the same period， 54 healthy individuals were recruited as the control group in hospitals and society. This study compared the thyroid function indicators among three groups of subjects， the Hamilton Depression Scale （HAMD） and Hamilton Anxiety Scale （HAMA） scores between two groups of patients， and the cognitive function of the three groups of subjects using the Measurement and Treatment Research to Improve Cognition in Schizophrenia （MATRICS） Consensus Cognitive Battery （MCCB）. Pearson correlation was used to examine the correlation among MCCB scores and HAMD scores，HAMA scores and thyroid stimulating hormone（TSH） in two groups of patients， and multiple linear regression was used to analyze the influencing factors of cognitive function in both groups of patients. Results In the bipolar disorder depressive episode group and MDD group， the HAMD and HAMA scores of patients were higher than those in the control group ［21.0 （19.0， 27.0） vs 22.0 （19.0， 25.0） vs 0 （0，0）， 15.0 （8.0， 18.0） vs 15.0 （9.0， 16.0） vs 0 （0，0）］， and the thyroid stimulating hormone （TSH） was lower than that in the control group ［（2.38±1.85） vs （3.29±2.37） vs （3.35±0.81） μ IU/ml］， and the differences were statistically significant （P＜ 0.05）. The scores of speed of processing， working memory， verbal learning， visual Learning， reasoning and problem-solving， and social cognition of the bipolar disorder depressive episode group and MDD group were lower than those of the control group ［（29.40±13.85） vs （37.87±10.99） vs （50.24±8.29）， （31.80±9.24） vs （30.49±8.42） vs （46.72±9.56）， （31.24±8.61） vs （32.89±9.04） vs （44.13±7.78）， （24.00±8.47） vs （25.96±6.64） vs （42.96±6.91）， （35.16±11.35） vs （40.02±9.84） vs （51.50±7.09）， （43.64±11.20） vs （46.53±10.54） vs （52.33±9.85）］， and the differences were statistically significant （P＜0.05）. The speed of processing， reasoning and problem-solving scores of the bipolar disorder depressive episode group were lower than those of the MDD group， with statistically significant differences （P＜ 0.01）. In bipolar disorder depressive episode group， the HAMD score was negatively correlated with scores of the speed of processing， working memory， verbal learning， reasoning and problem-solving （r=-0.456 to -0.315， P＜ 0.05）， the HAMA fscore was positively correlated with the score of verbal learning （r=0.441， P＜ 0.05）， and the differences were statistically significant. In the MDD group， the HAMD score was negatively correlated with speed of processing and visual Learning， and the differences were statistically significant （r=-0.568， -0.336； P＜ 0.05）. There was no correlation between cognitive function and TSH levels in patients in the bipolar disorder depressive episode and MDD groups （P ＞ 0.05）. Multiple linear regression showed that the HAMD score was a factor affecting the speed of processing， working memory， reasoning and problem-solving， and verbal learning of patients with bipolar disorder depressive episodes with a statistical difference （P ＜ 0.05）， while the HAMA score was a factor influencing verbal learning in patients with bipolar disorder depressive episodes. The HAMD score was a factor affecting the speed of processing and visual learning of MDD patients， while a history of alcohol consumption was a factor influencing the reasoning and problem-solving with a statistical difference （P＜ 0.05）. Conclusions Patients with bipolar disorder Ⅰ depressive episodes and MDD have significant cognitive impairment， with the former having more severe impairments in processing speed and problem-solving skill， and lower TSH levels. Symptoms of depression， anxiety， and alcohol consumption may be influencing factors on cognitive function. Early assessment and intervention of cognitive function are necessary in clinical diagnosis and treatment， and regular monitoring of thyroid function and evaluation of emotional scales are needed to assist diagnosis and treatment and promote cognitive function recovery.