This paper presents a fully integrated charge pump with gate voltage optimization for energy harvesting applications. In this paper, the benefits of powering a multistage charge pump with its own output are analyzed and the optimization of gate voltage are discussed. With optimized gate votlage, a 5-stage and a 3-stage charge pump are implemented with 180nm CMOS technique. Down to 0.12V/0.13V startup votlage are achieved by the propsoed 5/3-stage design correspondingly. Comparing with a similar 3-stage linear charge pump in previous state-of-the-art research, a 20% peak power conversion efficiency improvement is achieved by the proposed design under the same 0.18 V input voltage.