Objective To explore the effect of cranial electrotherapy stimulation (CES) on cognitive function for old patients with mild cognitive impairment.Methods A total of 40 old inpatients with mild cognitive impairment in 2018 and 2019 were randomly divided into control group (n = 20) and treatment group (n = 20). Both groups accepted routine medication (without cognitive drugs), and the treatment group accepted CES in addition, for eight weeks. They were assessed with modified Barthel Index (MBI) and Montreal Cognitive Assessment (MoCA) by two researchers single-blind before treatment, and four and eight weeks after treatment. The control group accepted free CES after trial.Results For MoCA score, the main effect was significant in time (F = 5.603, P = 0.007), not significant in group (F = 2.160, P = 0.150), and the effect of interaction was significant (F = 9.160, P < 0.001), which was more in the treatment group than in the control group. For MBI score, the main effects were not significant both in time (F = 0.322, P = 0.726) and in group (F = 0.009, P = 0.925), nor the effect of interaction (F = 0.322, P = 0.726). No adverse reactions occurred during CES intervention.Conclusion CES may be effective on mild cognitive impairment in old patients.