In view of complexity and response delay of shunt active filter harmonic current detection, constant power control for shunt active filter is introduced based on instantaneous power theory. Through the inverter active power and reactive power decoupling, separates the compensated power component and applies the space vector modulation algorithm to realize system constant power control. The Matlab/Simulink simulation shows that the strategy is effective on inhibiting AC harmonic current, compensating reactive power, outputting stable active power as well as improving the system steady-state performance, which verifying the control correctness and validity.