Metro is an important urban power load, the random fluctuations of the metro power will cause the city''s power grid voltage fluctuations and other issues. Firstly, based on the equivalent circuit of the metro power supply system, the mechanism of the power fluctuation is analyzed. Then, according to the measured metro power waveform , the moving average method is used to separate the metro power’s minute-scale component. After that, t location-scale distribution, logistic distribution, normal distribution and extreme value distribution are chosen to fit the minute-scale component of the main metro substation power, which contains daily power curve and quarterly power curve. Finally, the probability distribution characteristic is obtained. The results show that the power fluctuation of the metro is mainly concentrated in the driving time, while it is small in the break time at night. The fluctuation component of the metro power can be approximately fitted by the t location-scale distribution, and the fitting effect of Logistic distribution and normal distribution is better only when the voltage flicker exceeds the standard.