Abstract: Aimed at the drawback of the conventional consolidation analysis method, the ground with partially penetrating sand drain could be viewed as double-layered ground based on the equivalent calculation method.Then the mechanism of consolidation of double-layered ground improved by surcharge preloading method is analyzed, and a method to calculate the excess pore water pressure in the double-layered ground under the condition of staged loading or unloading is deduced. Considering the influence of time factor, a new consolidation calculation method considering the structure character of soil is provided. Finally, the above method is applied in the analysis of a case history, it is found that surcharge preloading method is disadvantageous to reduce the post-construction settlement of deep and thick soft ground and the post-construction settlement is mostly caused by the compressiont of the sub-soil which consists of 74 percent of the total post-construction settlement.