A photonic microwave down-conversion system and method. The oscillation loop includes an electro-optic modulator, optical narrow-band notch filter, photodetector and electrical combiner which has the function of generating the local oscillator signal. The frequency of the local oscillator signal can be flexibly adjusted by changing the lightwave frequency output from the wavelength tunable laser. The phase modulation function of the electro-optic modulator enables no RF signal output from the down-conversion photodetector It can not only down convert the RF signals with different frequencies to the intermediate frequency signals with the same frequency, but also can down convert the RF signals with the same frequency to the intermediate frequency signals with different frequencies. This can effectively overcome the limitations of complicated structure, poor RF isolation and difficulty in flexible tuning for microwave down-conversion system in the prior arts.