This paper present a novel solar thermal desalination system based on PEPD process, the so called PEPD is a desalination process feature with psychrometric energy process, namely the interaction process between air and water/salt water. The desalination process just use solar or waste heat to get the salty water evaporated into the air and the water vapor in the air, and then condensed to get pure water and also release the heat for salt water evaporation, so the energy is reused and very little thermal energy is required for the process. The PEPD process could be one stage or multiple stages, the one stages system thermal COP is about 3.5 and the two stage system thermal COP is about 6.