The Venus atmospheric environment was described with mathematical model.The Venus balloon thermodynamic model was established.Then

the floating process of a helium prototype balloon

of which the floating altitude was designed as 35km

was numerically designed by this model.The results show that when the Venus balloon is released at the altitude between 30km and 40km

it can reach floating altitude quickly and keep long-term floating

the balloon system should endure the temperature between 440K and 520K

its instantaneous maximum speed is less than 10m/s.This research would provide basis for the further development of Venus balloon further development.