Based on the analysis of the high-altitude balloon thermodynamic environment, a coupling dynamic model was established to describe its thermodynamics and kinetics. The model was used to simulate the ascending and floating processes of a high altitude balloon. The results show that the inner helium temperature will present "supercool" during its ascending process, and "supercool" is more pronounced during the stratosphere region, with the inner helium temperature 19 K below the surrounding air temperature. Because of "supercool", its ascending velocity profile take on double "V" shape. The inner helium temperature presents "superheat" during the daytime floating process due to strong sun radiation and weak convection with air, and the inner helium average temperature is 39 K higher than that of the surrounding air, with 648.8 Pa super-pressure. The ascending velocity profile and inner helium temperature profile of the numerical simulation good agreement with experimental flight data shows that the established model is accurate.