Abstract: During the service process of the last-stage blades of a steam turbine, the inlet edges often face the risk of cavitation damage, which leads to the loss of blade dimensions and has an adverse impact on the overall operation of the unit. Laser phase transformation hardening has great potential in improving the strength, wear resistance and fatigue resistance of materials. Aging treatment based on laser phase transformation hardening can further enhance the mechanical properties of materials. However, currently, there are few studies on the influence of laser phase transformation hardening and aging treatment on the cavitation-erosion resistance of 17-4PH steel used in steam turbine blades. Therefore, in this paper, taking 17-4PH steel as the material, the microscopic tissue distribution, hardness relationship, cavitation resistance and cavitation damage mechanism of 17-4PH steel under three treatment processes, namely laser phase transformation hardening, laser phase transformation hardening and aging treatment, and traditional solution aging treatment were studied. The results showed that after laser phase transformation hardening, a primary hardened zone mainly composed of coarse lath martensite and a secondary hardened zone mainly composed of tempered martensite were formed. The average surface hardness of the primary hardened zone was 352 HV_0.5, which was 12.8% higher than that of the matrix, and the average surface hardness of the secondary hardened zone was 381 HV_0.5, which was 22.1% higher than that of the matrix. After aging treatment of laser phase transformation hardening, the uniformity of the microstructure was improved, an obvious lath martensite structure was formed, and the average surface hardness reached 451 HV_0.5. The structure of the traditional solution aging treatment was mainly composed of lath martensite and ferrite, and the average surface hardness was 410 HV_0.5. Compared with the traditional solution aging and laser phase transformation hardening samples, the laser phase transformation hardening and aging sample had obvious diffraction peaks of ε-Cu and austenite. The cumulative mass loss of the sample treated by laser phase transformation hardening and aging was the lowest, only 60.28 mg, which was 55.1%, 31.8% and 10.5% less than that of the untreated sample, the laser phase transformation hardening sample and the traditional solution aging sample respectively. During the cavitation process of each sample, the material loss mechanism was spalling caused by crack propagation. However, after aging treatment, the precipitated ε-Cu increased the resistance to dislocation movement and restricted the crack from propagating inward. As a result, the surface damage of the laser phase transformation hardening and aging sample was relatively small, and the mass loss was the minimum.