Abstract: Based on the experimental phenomena, the effect of temperature distribution on natural circulation capacity is analyzed theoretically, and the expressions about the influence function of distribution are obtained. It is indicated that the heat transfer between upward section and downward section and the parabola temperature distribution in the heat source and heat sink have certain but not heavy influence on the natural circulation capacity in the natural circulation loop. The deviation of natural circulation caused by these two reasons is a function of geometric structure of natural circulation system, temperature increase in the upward section, the thermal expansion coefficient and the type of the parabola temperature distribution, and the effects of different reasons are separated. When the temperature increases or decreases in the upward section or downward section ΔT＜1~20 ℃, the deviation of natural circulation capacity is limited in ε%＜0.2%~4%.