Objective To evaluate the clinical value of P300 and total cerebral small vessel disease （CSVD） score in detecting the degree of cognitive impairment in patients with CSVD， and to explore their matching degree with cognitive function screening scale. Methods A total of 72 patients diagnosed with CSVD in the Department of Neurology of Lianyungang Oriental Hospital from October 2018 to June 2020 were included， and 34 normal physical examinees matched by age and gender were included as the control group. Mini-mental State Examination （MMSE）， Montreal Cognitive Assessment （MoCA）， P300 and total CSVD scores were examined within one week for all the subjects included. According to the cognitive function screening scale （MMSE， MoCA）， CSVD patients were divided into cognitive impairment group and non-cognitive impairment group. The relationship between MMSE and MoCA scores， and P300 latency， P300 amplitude， and total CSVD score among the three groups was analyzed. Results The latency of P300 recorded at the central point （CZ） in cognitive impairment group and non-cognitive impairment group was longer than that in control group （P＜0.001）.The latency of P300 recorded in cognitive impairment group was longer than that in non-cognitive impairment group［（397.471±35.911） ms vs. （342.584±14.502） ms，P ＜ 0.001］. The total CSVD score of cognitive impairment group and non-cognitive impairment group was higher than that of control group （P＜ 0.001）， and the total CSVD score of cognitive impairment group was higher than that of non-cognitive impairment group （P＜ 0.001）. Correlation analysis results showed that MMSE and MoCA scores have strong negative correlations with P300 latency （r=-0.768，P＜ 0.001；r=-0.824，P＜ 0.001） and total CSVD score （r=-0.816，P＜ 0.001； r=-0.896， P ＜ 0.001）. When the CZ of P300 latency was 348.4 ms， the sensitivity and specificity of CSVD were 94.3% and 83.1%， respectively. When the total CSVD score was 2， the sensitivity and specificity of CSVD were 82.9% and 97.2%， respectively. Conclusions P300 latency and total CSVD score may be the indicator of cognitive impairment in patients with CSVD for the early stage， and can reflect cognitive function more objectively than MMSE and MoCA.