This paper details the relationship between urban disease and city size. It advances the research of the theory of optimal city size generally. Then it takes Beijing as an example，analysis the historical evolution and its control of population size，which shows that since 1949 Beijing has been controlling the scale of city strictlyas its goal，but the effort was not successful. The fourth part takes the utilization of water resources and city traffic for example，illustrated that the shortage crisis of both natural resources and public services are not because of the expansion of population size. The theory，history and case analysis show that，there is no necessary and direct connection between the urban disease and city size. Urban issues of negative externalities need regulation policy；urban issues with macroeconomic and social transformation，requires fundamental changes. In short，dealing with “urban diseases”，China need to go beyond the scale of thinking，in order to achieve inclusive development，let more people enjoy the benefits and convenience of urbanization.