Photonic crystal surface emitting lasers (PCSELs) have the advantages of high power and high beam quality because they can achieve large area single mode lasing. They have important application prospects in optical communications, laser radar, laser printing, laser display, and laser processing. This paper focuses on the analysis of the band-edge lasing principle and threshold gain of the photonic crystal laser with in-plane multi directional distributed feedback effect, and combines the semiconductor laser rate equation to deduce the output optical power formula of PCSELs, and also gives some effective methods to increase the optical power of PCSELs. The research results can provide theoretical guidance for the development of high performance PCSELs.