Provided is a method of calculating the power consumption mean value of a multiple transformer parallel power supply system, comprising the steps of: S1, calculating line and transformer impedance; S2, calculating the mean value and variance of a line apparent power, active power and reactive power changing according to a normal distribution law; S3, calculating the mean value and variance of a transformer apparent power, active power and reactive power changing according to a normal distribution law; S4, calculating the mean value and variance of a load apparent power, active power and reactive power changing according to a normal distribution law; S5, calculating the loss mean value of the line active power and reactive power; S6, calculating the loss mean value of the transformer active power and reactive power; and S7, calculating the loss mean value of the active power and reactive power of the power supply system composed of parallel NT transformers. The invention provides the method of calculating the power consumption mean value of a multiple transformer parallel power supply system, meanwhile taking regard of the uncertainty and randomness of a power grid operation mode and a load, and making calculating results relatively accurate.