Preconsolidation Pressure is an index of deciding the stress history of a soil layer; and it is also an important computation parameter in settlement analysis dealing with soils which have different stress histories. It is very significant to determine preconsolidation pressure conveniently and precisely. A new method of determining preconsolidation pressure named as mathematical model method based on theoretical research and test analysis. Results of tests for soil samples taking from Runyang great bridge demonstrated that the application of the method is recommendable.