Abstract: In order to ensure the safety of space radioisotope heat unite in fire accidents, environmental simulation tests and numerical simulation analysis on the radioisotope heat unite in fire accidents were performed. The method of environment test and apparatus design was proposed, and numerical analysis on heat transfer characteristics of the product during the test was carried out. Based on the numerical results, the fire test on a type of radioisotope heat unite was carried out. The results show that the preheating temperature significantly affects thermal response characteristics of isotope heat source, and the emissivity of the product surface mainly affects the temperature of the ablation layer. In addition, the temperature of the ablation layer of the product rises rapidly in fire accidents, and the temperature rise rates of each layer in the launch site accident are higher than those in the transport accident, but the pellets are still at a safe temperature in fire accidents. The test shows that the flame has the shape of plume and has the characteristics of large scale-low frequency disturbance character. Finally, the surface temperature of product drops to 180 ℃ and the product has a complete structure 30 min after the flame extinguished.