As a high efficient two-phase heat transfer device, the loop heat pipe implements far distance transfer of heat mainly by using the gaseous phase change of a working fluid. Therefore, the nature of working fluid and its filling ratio have a great influence on the performance of the loop heat pipe. Several key factors including figure of merit, fluid filling rate and gravity that have influences on the performance of loop heat pipes are studied theoretically and experimentally. The loop heat pipes are optimized. By using a pulse-tube cooler as the cold source, the application performance of it integrated with the loop heat pipe is studied. This research has laid the theoretical and experimental foundation for the space application of loop heat pipes.