In order to study the relationship between the diffusion of the accidental pollutant diffusion in urban building blocks and the density of buildings and the location of pollution sources, a building model has been constructed for seven regularly arranged urban block buildings with a density of 10%, 25% and 40% respectively, followed by a simulation of the pollutant diffusion in different source locations by using a computational fluid dynamics (CFD) software Fluent. The results show that the air flow in all directions of urban blocks performs the best with a small building density. When the density of the block reaches a certain level, it will also promote the diffusion of the polluted gas in the direction of flow, with the density of the building arrangement exerting a significant impact on the distribution of pollutants. When the pollutants are located in the front of or in the middle of buildings, the best ventilation effect can be obtained with a density of the middle buildings（λ=0.25）and a density of the low buildings（λ=0.1）respectively.