Laser self-fusion welding was used to weld Q235 ordinary carbon steel and 304 austenitic stainless-steel dissimilar steel, and the effects of different welding parameters on microstructure and mechanical properties of dissimilar steel welded joints were studied. Experimental results show that high quality welding seam and small heat affected zone can be obtained due to the high concentration of laser energy density. According to the element content obtained from the direct reading spectrum, and refer to the Schaeffler diagram, it can be inferred that the microstructure of the weld area is composed of martensite. Metallographic microscope and scanning electron microscope were used to verify that the weld seam is mainly composed of lath martensite. The microhardness tests show that the microhardness of weld zone is higher than that of base metal on both sides. Under the condition of sufficient heat input and complete penetration of welded joint, tensile fracture occurs on the side of Q235 steel of base metal, which indicates that the strength of welded joint is higher than that of base metal if the welding heat input is insufficient, there will be incomplete fusion defects and even air holes in the weld joint, and the tensile fracture of welded joint occurred in the weld zone.