In the high flux engineering test reactor (HFETR), the change of gas pressure in
3He loop would introduce reactivity to the reactor, and then affect the operation safety of the reactor. The reactivity change of the variable power rig was calculated by MCNP code in this paper, and the superimposed accidents of the loss of pressure in
3He tube and the withdrawing of the control rods were analyzed by RELAP5 code. The results showed the irradiation tests conducted in rig do not have effects on the normal operation of HFETR. Even when the loss of pressure in
3He tube and the withdrawing of the control rod happened at the same time, the safety of HFETR was guaranteed.