The body temperature of pilots wearing anti-G garments becomes uncomfortably or even dangerously high in moderate to hot environments. To reduce the body heat stress, thermal characteristics and thermal protection performance of anti-G garments should be studied systematically, but up to now there is no related research to simulate the anti-G garment thermal protection performance. First, a human thermoregulation model is established which can analyze blood convective heat exchange and the central blood temperature fluctuation, according to this model the heat and mass transfer processes of a thermal system are simulated which consists of human body, an anti-G garment, and the surrounding environment, then on the base of this thermal system simulation influences of clothing permeability, thermal resistance and bladder coverage on anti-G garments are analyzed. Calculation of the human thermoregulation model is carried out by using finite element method. Experiments are conducted in 35 °C and 40 °C climate chambers, and model built above is well validated by these experiments. Model simulation reveals that low thermal resistance and high permeability of anti-G garments reduce body heat stress in moderate environments. In hot environments, however, anti-G garments with lower thermal resistance increase body heat stress. The body heat stress rises significantly as the bladder coverage area of anti-G garments increases. Anti-G garments thermal parameters can be appropriately designed by using the model built in this paper to ensure body physiological requirements.