Studies on the microstructure and properties of butt joints by laser welding 1.5 mm thickness DP1000 dual phase advanced high strength steel used tensile test, microhardness test, scanning electron microscope (SEM) and optical microscope(OM) are presented. The reason of the decrease of joint′s strength and ductility is discussed. The results show that a serious softening has occurred in heat-affected zone (HAZ) of the welding joints, and the most serious locates in the tempered zone and the incomplete crystalline zone of HAZ. The strength of welding joints decrease by 10%~15%, and the ductility of welding joints exceed 60%. The main reason of the joint′s strength descent is the existence of tempered martensite which is obtained in the welding process, and the tempered martensite′s strength is lower than the quenching martensite. The main reason of the joint′s ductility descent is the uncoordinated tensile deformation of different joint subareas, and the deformation mainly takes place on the softened zone of HAZ. The welding fusion zone, the base metal and other zones of HAZ hardly occur deformation.