In order to understand the basic characteristics of the heat transfer of natural convection of water near the maximum density in a horizontal annulus, twodimensional numerical simulation was conducted using the finite volume method. The outer wall temperature was maintained at 0 ℃, and the temperature of inner wall at 1 ℃ to 10 ℃. The results show that the heat transfer coefficient at the inner wall first decreases with increased annulus width, then increases, and then constantly decreases with the increased inner radius in the region of calculation. When the temperature difference is below 4 ℃, the heat transfer coefficient increases with increasing temperature difference. When the temperature difference exceeds 4 ℃, it decreases with the increase of the temperature difference. Finally, the formula of heat transfer at the inner wall was obtained by using stepwise linear regression.