The invention relates to a constant power limiter including a control circuit and a power circuit thereof. The control circuit includes a voltage testing circuit, a current testing circuit, a zero cross testing circuit, a load drive circuit and a microcontrol unit. The microcontrol unit receives the output signals of the voltage testing circuit, the current testing circuit, the zero cross testingcircuit and outputs control signals to the load drive circuit to control the power of the alternating current to load. The control circuit also includes a bypass circuit connected with the output endof the microcontrol unit and used for bypassing the alternating current to load directly when the control circuit has fault. The invention can limit load power of consumers or electric apparatuses ina predetermined range, save electric energy, avoid overload of electric network and energy source waste. The design of the bypass circuit can supply power for consumers or electric apparatuses normally without influence on normal operate thereof when the constant power limiter has fault.