The paper analyzed the influence of pipe spacing on natural convection of cylinders so as to provide theory reference for solving thermal diffusion problem of river.The result has certain application value.The model of a pair of horizontal cylinders in a channel is abstracted. It introduced SIMPLE to simulate the natural convection under different longitudinal pitch. The results showed that with the increase of pipe spacing S,the flow between two horizontal cylinders become stronger.The heat transfer around the upper cylinder is enhanced obviously, the more the Ra,the more evident the enhancing effect.In addition,the paper investigated the influence of longitudinal pitch ratios on chimney effect.The volume flow rate in the channel increases with the increase of S. The chimney effect is strengthen.