The deformation performance of support wall for foundation pits was analyzed based on the practice of a station on Shanghai Subway Line M8. It was found that the pit displacement can be reduced obviously during the excavation of pits by decreasing appropriate amount of excavation space and time every step and by shortening free exposure time of pit’s wall, making use of tau-effect laws. The goals of controlling pit deformation scientifically can be reached by using the laws of tau-effect while excavating the foundation pits and sufficiently stimulating the potential self control of deformation of soft soil.