Objective To investigate the effects of high-definition transcranial direct current stimulation（ HD-tDCS） on psychiatric symptoms and brain functional activity in patients with chronic schizophrenia. Methods This study used a randomized， sham-controlled， double-blind design. A total of 65 patients with chronic schizophrenia were recruited at the Third People's Hospital of Foshan between January and June 2021 and were randomly assigned to the HD-tDCS group（ n=32） and the sham group（ n=33） in a 1∶1 ratio using computer-generated randomization. After 10 days of intervention and the exclusion of those with detachment（ n=9） or poor image quality（ n=11）， the subjects who finally completed the trial and were included in the analysis were 24 in HD-tDCS group and 21 in sham group. HD-tDCS group received HD-tDCS at 2 mA for 20 minutes， while sham group received only 30 seconds of sham stimulation. The patients' psychiatric symptoms were assessed by the Positive and Negative Syndrome Scale（ PANSS）， and their brain functional activity ［amplitude of low frequency fluctuations（ ALFF） and functional connectivity（ FC）］ was evaluated using restingstate functional magnetic resonance imaging（ rs-fMRI）. PANSS scores and imaging indicators were compared before and after intervention in each group， and the difference in PANSS scores and imaging indicators between the two groups before and after the intervention was compared. Partial correlation analysis was used to assess the correlation between post-intervention changes in psychiatric symptoms and changes in ALFF/FC. Results The positive symptom score［ （13.5±6.44） vs.（ 16.04±7.51）］， general psychiatric symptom score［ （31.04±9.09） vs. （35.04±10.88）］， and total symptom score［ （59.96±16.07） vs.（ 68.79±23.51）］ in HD-tDCS group decreased after intervention， and the differences were statistical（ t=2.25， 3.30， 3.13； all P＜0.05）. After intervention in HD-tDCS group， ALFF increased in the left posterior cerebellar lobe（ CPL）［ （-0.22±0.21） vs.（ -0.37±0.14）］， while FC enhancement was observed in the left CPL compared to itself［ 0.42（0.37，0.50） vs. 0.21（0.17，0.27）］， left posterior cingulate cortex（ PCC）［ 0.23（0.19，0.27） vs. 0.08（0.05，0.10）］， and left hippocampus［ 0.21（0.17， 0.25） vs. 0.06（0.05，0.08）］， and the differences were statistical（ t=-5.27，-4.29，-4.29，-4.29； all P＜0.05）. Correlation analysis showed that in HD-tDCS group， the rate of change in FC between left CPL and left PCC before and after the intervention was positively correlated with the rate of change in general psychiatric symptom scores and total symptom scores， and the difference was statistically significant（ r=0.46，0.48；all P＜ 0.05）. Conclusions HD-tDCS significantly improves psychiatric symptoms and cerebellar-related neural network activity in patients with chronic schizophrenia.