In the process of urban development, a large number of areas under overpass has become the integral part of urban renewal at the stage of lost space and urban repair in two ways. This paper firstly combs the lost space in the urban development process and the way of updating, and then an empirical research focusing on the Yang Gongqiao interchange has been carried out based on the investigation of the informal development of the lost space in the Chongqing overpass. This paper analyzes the user's behavior demand and concludes the spatial operation system, explores the blind spot of the current planning policy. Finally, it proposes specific suggestions for the planning strategy and governing measures of the informal space.