A novel and simple reference light power calibration method is proposed, which has the advantage of frequency discriminator. The difference between light power of the total output of electro-optic modulator and the intended reference light power is detected at each frequency point, and then used as the feedback signal to control the output power of microwave source to calibrate the reference light power. The feasibility of the method is analyzed theoretically and verified through simulations and experiments. It is demonstrated that, after the calibration, the maximum error between the reference light power and intended power is reduced to one-third of that observed before calibration.