The thermal and dynamic properties of a pumpkin-shape super-pressure balloon with load tendons during ascent were studied. Thermal environment and forces were analyzed, and the thermal and dynamic models were proposed, and a simulation program was developed by using MATLAB. A simulation of a super-pressure balloon flight was conducted, the flight altitude, the ascent rate and temperature were analyzed during ascent. The results suggest that the thermal environment as well as the dynamic properties exert a great influence on the temperature of the Helium inside the balloon, the ascent rate and Helium temperature interact with each other, and both are relevant to the free lift ratio. The comparison between the simulation results and flight data reveal that the two sets of data match well. The theoretical analysis and simulation model are helpful in the super-pressure balloon design and mission planning.