For the operating characteristics of HFETR,the numerical model of HFETR was established by RELAP5/MOD3 to analysis the maximal natural circulation capacity. Combining with the reactor running condition,the influence of the system pressure was analyzed by ascending power in step method and the pool water temperature on natural circulation characteristics was analyzed by integral power method. The results show that the natural circulation capacity are 0.9 and 2.0 MW separately under low pressure and high pres...