The principle and technique characteristics of phase-modulation to intensity-modulation conversion (PM-IM), which is widely used in microwave photonic system, are analyzed. Categories of PM-IM are summarized. The applications of microwave photonic filter, ultra-wideband (UWB) system, all-optical microwave generation, microwave frequency conversion and instantaneous microwave frequency measurement are introduced in detail, and the advantages of PM-IM are discussed. At last, it is pointed out that the research directions of PM-IM are improving rate and precision, reducing insert loss and improving system reliability.