In this study, the phase stability of martensite in the heat-affected zone of 17-4PH stainless steel during laser cladding is investigated through a combination of phase field simulation, finite element analysis, and phase transformation experiments. Based on the JMAK model and experimental data, the evolution of secondary phases during the high-temperature stage of the cooling process in laser cladding is analyzed. The effects of this evolution on the matrix chemical composition and yield strength are examined. The coefficient of the phase-field model is optimized by combining martensite transformation experiments with a BP neural network. A quantitative elastoplastic phase field model that involves the evolution mechanism of secondary phases is established. Phase-field simulations show that zones with higher peak temperatures in the heat-affected zone exhibit higher martensite transformation start temperatures (Ms) and faster martensite transformation rates. The simulation results are in good agreement with experimental results. The phase-field simulation in this study reveals that elastic strain energy is the dominant factor controlling the thermal stability of martensite transformation.