Theory analysis shows that the copper molten amount has great influence on the joint of copper-steel welding for the large difference in physical and chemical properties between copper and steel. Considering the high laser reflectivity of copper, laser welding for copper-steel joining based on slope butt joint and laser beam offsetting to the steel side is researched. Copper-steel butt joints are welded under three different welding parameters. The observation of microstructure and the measured mechanical properties show that joint quality strongly depends on the molten amount of copper in weld. When the copper molten amount is small, the interface between the copper and the weld is clear, and the joint has no defect and shows good mechanical properties. When the copper molten amount is large, there is no clear interface between the copper and the weld, pores and cracks are observed in weld, and the mechnical performance of joint is poor. Experiment results indicate that with the laser beam offset of 0.8 mm. A low fusion ratio of copper and steel is achieved and a defect free joint is obtained.