To analyze the influence of building density in urban areas on the transport process of dissolved pollutants, taking conservative dissolved pollutants as the object, a two-dimensional mathematical model for rainfall runoff and pollutant transport in the building area was established, and the effects of different building densities on the mass concentration, transport rate and cumulative transport process of dissolved pollutants were simulated and analyzed. The simulated results show that when the building density ranges from 4% to 16%, the flow rate increases relatively quickly in the middle stage of runoff confluence. With the increase of building density, the water depth downstream of the building gaps first increases and then decreases, and reaches its maximum value at a building density of 36%. The mass concentration of pollutants at the regional outlet generally decreases gradually, while the pollutant transport rate first increases and then decreases. In the case of staggered building arrangements, the upstream pollutants are intermittently transported downstream through the gaps between buildings. When the building density is greater than 9%, the mass concentration and transport rate of pollutants at the regional outlet exhibit multiple fluctuates before the runoff reaches its maximum. With the increase of building density, the transport rate of pollutants decreases and the peak occurrence time is delayed. The final cumulative transport proportion of pollutants decreases with the increase of building density.