Energy harvesting technology is a novel research topic in these years. The energy harvesting circuit can harvest micro-level energy from vibration, RF signal, heat in the environment and provide DC voltage for many applications. In the energy harvesting circuit a charge pump which can convert micro-level AC signal to DC voltage is very important. Generally, a charge pump is design by many diodes or transistors and the AC-DC efficiency will be 50~70% only when the input power is larger than 10 dBm. In order to convert micro-level input power of energy harvesting circuit, we need to research and redesign the charge pump for input power smaller than -10 dBm and try to improve the poor efficiency of charge pump.