Electroconvulsive therapy (ECT) is an effective therapy for many psychiatric illnesses. However, intracranial occupying lesions are a relative contraindication to ECT. Arachnoid cysts are benign, congenital, and space-occupying lesions. Our study aimed to evaluate the efficacy and tolerability of ECT in psychiatric patients with arachnoid cysts. We retrospectively identified 62 psychiatric patients with arachnoid cysts; 43 of them underwent ECT and 19 did not. Their conditions were assessed by CGI-S and different scales depending on different diagnoses (PANSS for schizophrenia; HAMD for depression; YMRS for bipolar disorder). The side effect was assessed by TESS. Significant differences were shown in the reduced scores of the CGI-S between patients who underwent ECT and those who did not (p = 0.001), while, at the same time, there was no significant difference in their TESS score (p = 0.297). The current study found that ECT is an effective and tolerable therapy for psychiatric patients with arachnoid cysts.