A method for controlling start time series by stages adopted for use on a power supply consisting of a main power system and a standby power system. The power supply includes an energy storage unit to store a judgment power. The method mainly includes: setting a start voltage, setting a boosted voltage, getting a judgment power voltage and detecting whether the judgment power has reached the start voltage or the boosted voltage, and alternately starting the standby power system and the main power system. Thus the main power system and the standby power system are started at different time series. In terms of the energy storage unit the load increases by stages. Therefore the energy storage unit can be charged to increase the judgment power voltage and increase loading by stages. As a result, inrush current also rises by stages and can be harnessed as desired.