In allusion to the insufficiency of capacitor bank single-phase fault protection at present, this paper puts forward a combined voltage criterion protection method by comparing three-phase ground voltage to distinguish the single-phase earth fault. Aiming at constructing combined voltage criterion, voltage value comparison method and voltage phase comparison method are analyzed theoretically and a protection setting calculation method is given. Through analyzing and comparing the advantage and disadvantage of the two methods, voltage value comparison method is recommended to construct the combined voltage criterion. Furthermore, the feasibility of combined voltage criterion is verified by simulation. Since the protection will act only if all the three phase voltages are satisfied, the combined voltage criterion has a better function to prevent protection misoperation caused by PT breaking. This work is supported by Science and Technology Project of State Grid Corporation of China (No. 52170218000M).