The deep penetration laser welding of 20Cr2Ni4A-20 g dissimilar steel is studied to increase the welding efficiency. The effects of laser welding parameters on the morphology of welding seam are investigated using 5 kW Slab CO2 laser. The tensile strength is tested by material testing machine, and the fracture morphologies are observed by scanning electron microscopy. The results show that the welding joint can be optimized by adjusting the defocusing distance, coaxial shielding gas flow rate, laser power and welding speed. The optimum process parameters of 8 mm thickness of 20 g and 20Cr2Ni4A dissimilar steel are: a laser power of 5 kW, a welding speed of 2.5 m/min, a shielding gas of 5 L/min He and 20 L/min Ar and a focusing distance of +2 mm. Under the optimum process parameters, the tensile strength of the welded joint is better than 20 g matrix, which is ductile fracture and the fracture presents dimple.