In order to solve the settlement calculation problem of saturated soft soil under different stress levels, the low stress component of the Nishihara model was analyzed and improved after considering the rheological properties. Through the Laplace transform and its inverse transform, a one-dimensional consolidation analytical solution was obtained under instantaneous loading condition. Based on the solution, the unified analytical solution was obtained in the method of integral under multilevel loading conditions. And the analytical solution was used in the settlement calculation for the test section of Dongting Lake Area soft soil embankment. It has been shown that the calculated settlement values of the analytical solutions under different stress levels have different change laws, which is in agreement with those of the field tests. At the early stage of consolidation, the calculated consolidation settlement rates are much lower than that of the elastic model under the same conditions. Therefore, when calculating the settlement of soft foundation, it is necessary to consider the influence of different stress levels on the settlement of soft foundation, and to consider lag effect caused by soft soil creep.