Using the balanced panel data of 283 cities at the level of municipality or above in China from 2014 to 2017, this study adopts the system generalized method of moments (SGMM) and panel threshold model to study the relationship among city size, urban-rural income gap and environmental pollution. The conclusions are as follows: (1) There is an inverted U-shaped relationship between city size and environmental pollution; (2) Urban-rural income gap can significantly aggravate environmental pollution; (3) Urban-rural income gap has a regulatory effect on the relationship between city size and environmental pollution. When the urban-rural income gap crosses the threshold, the intensity of environmental pollution caused by city size changes. Therefore, optimizing the city size and narrowing the urban-rural income gap will reduce urban environmental pollution.