Co based alloy coating with single pass on 42CrMo substrate was carried out by laser cladding, the influence of laser power, powder feeding rate, scanning speed and spot diameter on the geometric morphology and dilution ratio of cladding layer was investigated through orthogonal test. The results show that the laser power and scanning speed have great influence on the cladding height and width, and the laser power and spot diameter have the greatest influence on the dilution ratio. Combined with the comprehensive evaluation of dilution ratio and the ratio of cladding height and cladding width, the optimal process parameters combination is laser power 1 600 W, scanning speed 13 mm/s, powder feeding rate 24 g/min and spot diameter 3 mm.