In this paper, B1800 HS/DP1180 resistance spot welding joint is taken as the research object. The macroscopic morphology of the weld was observed by ultra-depth micro-scopy. The microstructure of the weld was studied via optical microscope（OM） and the hardness distribution of the joint was measured by the Vickers microhardness tester. The results show that the microstructure of B1800 HS is fine lath mart-ensite. The nugget of the spot welded joint is good and asymmetric and the microstructure is thick lath martensite. The nugget diameter is 6 500.32 μm. The nugget depth is 1 868.17μm, the width of the heat affected zone is 692.63 μm and the electrode indentation depth is 216.04 μm,which conforms to the industrial standards of Japan and Germany. The weakest part of the joint is the subcritical heat affected zone,the microstructure is tempered martensite and the hardness is the lowest,271 HV0.3,where cracks are usually produced.