Abstract: The physical and mechanical indexes of residual soil in natural state and in saturation state on seven sites were obtained by the soil tests,based on the formulas of code for design of building foundation and the formulas of Terzaghi ultimate bearing capacity foundation,the influence on the bearing capacity of residual soil due to the state of ground water are analyzed,the decreasing range of the bearing capacity of residual soil after soaking by water may descend 2.8 to 58.8 percent.The results of twelve loading tests indicate that,the characteristic value of subsoil bearing capacity of the residual soil after short time and long time soaking by water decreased obviously,the decreasing range can reach 11 to 57.5 percent.