To explore the efficacy, adverse reactions, feasibility, and acceptability of transcranial alternating current stimulation (tACS) treating drug-naive adult patients with major depressive disorder (MDD), and provide basis for further study with a large sample. The study was performed in the Neuromodulation laboratory, Department of Neurology of Xuanwu Hospital, Capital Medical University (Beijing, China) from July, 2017 to June, 2018. Thirty Eligible first-episode MDD outpatients were randomized 1∶1 to receive active tACS or sham intervention. The tACS was administered in a 40 minute, 77.5 Hz frequency, 15 mA session with one forehead (Fp1, Fpz, and Fp2, in the 10/20 international placement system, 4.45 cm×9.53 cm) and two mastoid (3.18 cm×3.81 cm) stimulation for 20 times in 4 consecutive weeks at fixed day time frame once daily from Monday through Friday, with weekends off (week 4), followed by 4 weeks with no tACS treatment (week 8). By utilizing the Hamilton rating scale for depression-17 item (HRSD-17) to assess the depressive severity of MDD patients, adverse events were administered by the treatment-emergent adverse events, the Young mania rating scale, and the self-made common questionnaire on cranial electrical stimulation. The primary efficacy outcome was the remission rate defined as HRSD-17 score ≤7 at week 8. Secondary outcomes included the rates of remission at week 4 and response at weeks 4 and 8. Safety was assessed by evaluation of adverse events. Also the proportions of participants accepting the intervention and this study procedure were evaluated at weeks 4 and 8. Thirty MDD patients completed the study, and both groups had no statistical differences on their demographic characteristics (0.05). At week 8, the active group had a remission rate of 10/15, which was higher than 3/15 in the sham group (0.05). Also, the remission rate (14/15) in the active group was higher than 5/15 of the sham group at week 4 (0.05). For the response rates, significant differences were found between groups at week 8. For safety, both groups showed no severe adverse events and no mania/hypomania. One participant per group had 2 times of tinnitus cerebri during the intervention days. All patients accepted the intervention and the study procedure. The pilot study indicated that tACS with 77.5 Hz and 15 mA may have a therapeutic effect on depressive symptoms. It is well-tolerated and safe, as well as feasible and acceptable for adults with MDD.