Abstract: Taking the pit of China Experimental Fast Reactor as the prototype, the finite volume method was adopted to conduct numerical simulation for a two-dimension laminar steady-state natural convection in vertical annular enclosed cavity. The results show that under the same conditions of Ra, the average Nu increases with the curvature, while decreasing with the increase of the aspect ratio (ratio of height and gap in annular enclosed cavity). But in lower Ra zone, the average Nu has little change with aspect ratio and the heat conduction is dominant. After the Ra exceeds critical value, the heat convection makes a greater impact in cavity and then gradually holds the dominant positions with the increase of Ra.