Abstract: Butt welding experiment of AH36 plate by fiber laser was carried out to study the effect of preheating on the microstructure of weld. The result showed that the microstructure of AH36 joint was dominated by lath martensite(M) and a small amount of granular bainite without preheating. With the increase of preheating temperature, the cooling rate decreased, resulting in the decrease of martensite number and the increase of bainite number in the weld. When the preheating temperature was 250^\circ \mathrmC, the microstructure was dominated by bainite and featherlike upper bainite. Microhardness of the joint was tested, and the result showed that the microhardness of the joint decreased with the increase of preheating temperature, which was consistent with the variation of martensite content in the weld.