The laser deep fusion welding test of 20mm thick 05Cr17Ni4Cu4Nb/HR-2 dissimilar stainless steel was carried out by using different defocus, and the microstructure of the welded joint was analyzed by optical microscope, scanning electron microscope and energy spectrum. The mechanical properties of welded joints were studied by microhardness tester, electronic universal material tester and metal pendulum impact tester. The test results show that the weld is well formed and no welding defects occur under different technological parameters. The microstructures of weld seams are all δ ferrite+austenite, but the morphology and quantity of δ ferrite change. With the increase of defocusing amount, the grain in weld zone is gradually refined, and the microhardness of weld zone is improved, which is higher than that of HR-2 base metal. In the three-point bending test, the bending positions all appear on the HR-2 base metal side, indicating that the welded joint has good plasticity. When defocusing is 10mm, vermicular ferrite in weld zone improves weld toughness and the impact toughness reaches the maximum.