To study the effect of laser power on laser cladding microstructure and hardness of 17-4PH stainless steel wire, ensure the best laser cladding power, and provide an evidence for 17-4PH stainless steel wire to apply for laser cladding, this paper carries out the laser cladding test for 17-4PH stainless steel wire on the surface of 27SiMn steel piston rod, observes and presents the microstructure of cladding layer under various laser powers by metallographic microscope and scanning electron microscope, and measures the hardness of cladding layer and substrate by hardness tester at different laser powers. It is found that the height of cladding layer is decreased from 1119 μm to 1006 μm, and the penetration depth of the substrate and the width of the heat-affected zone are increased with the increase of the laser power when the laser power is 1600 W, 1800 W, 2000 W and 2200 W, respectively. Its microstructure is mainly composed of shorter lath martensite without directionality. Besides, when the laser power is 2400 W and 3000 W, the height of the cladding layer, the penetration depth of the substrate and the width of the heat-affected zone increase as the laser power is increased, with the maximum up to 1119 μm, 310 μm and 638 μm, respectively. Its microstructure consists of lath martensite with directional orientation, and dense equiaxed crystals close to substrate. Meanwhile, the precipitation particles are increased with the increase of laser power. In addition, micro-hardness of the cladding layer and the heat-affected zone is higher than that of the substrate. Its hardness is up to 479.4HV0.2 as the laser power is increased. Therefore, it is concluded that 2600 W is the best laser cladding power, considering the influence of laser power on laser cladding structure and hardness of 17-4PH stainless steel.