With the rise of the underwater thermal jet, its momentum gradually decays and evolves into a plume. Using the conventional Reynolds-averaged method for simulation is prone to the problem of excessive eddy viscosity leading to the distortion of simulation results. In order to solve this problem, the outlet of an underwater thermal jet as the research object was taken, the calculation model of an underwater thermal jet by using Partially-Averaged Navier-Stokes (PANS) method was improved, the floating-up and diffusion process under static and moving conditions were simulated, and the floating-up and diffusion law of underwater thermal jet were analyzed. A fully transparent towing tank was built, and the floating-up diffusion images of the thermal jet measured by the PLIF method under static and moving conditions were compared with the numerical simulation results. The results show that the prediction model of underwater thermal jet based on PANS method has high accuracy, all errors are within 15 %, and most of the errors are controlled within 10%.