In order to control the thermal stability of water-laser coupling in water-jet guided high-power laser and to ensure the effective transmission of laser energy to the workpiece surface, the thermal loss problem of water-jet guided high-power laser was investigated. The finite element method was used to establish a numerical model of water-laser coupling, and ray tracing was used to simulate the temperature distribution of water jets with different incident laser power, jet velocity, and diameter. The model’s effectiveness was verified by experimental data, and continuous and pulsed laser temperature distribution was compared under the same power. The results show that the water jet’s temperature increases with the water-laser coupling length. When the incident laser power increases, the pressure of the coupling cavity decreases, the jet diameter decreases, and the temperature of the jet increases, which seriously affects the stability of the jet. Due to the water jet’s cooling effect, the pulse laser’s temperature difference with an average power of 300 W is about 5 ℃ compared with that of the continuous laser. The research results provide certain reference significance for controlling the thermal effect of water-laser coupling in water-jet guided high-power laser and improving the transmission efficiency of laser energy.