A method for detecting failure of soft start includes: connecting a variable frequency device to a load; detecting an energy storage unit at a first voltage level; generating an output current to the load; detecting the energy storage unit changed from the first voltage level into a second voltage level; generating a reference voltage value according to the output current and determining whether a voltage difference value between the first voltage level and the second voltage level is larger than the reference voltage value; determining whether a voltage peak-to-peak value of the energy storage unit is smaller than a set value by the controller if the voltage difference value is larger than the reference voltage value; and controlling the inverter circuit to stop outputting the output current to the load if the voltage peak-to-peak value is smaller than the set value.