Abstract: In order to investigate the relation between the shear strength and the effective normal stress of soil under the same water content, according to the methods for unloading shear strength of soil, the variation rules of shear strength of the soil under loading and unloading conditions are analyzed considering the stress relation between over-consolidated soil and normally consolidated soil. Formulas for the two true strength parameters of the true strength theory based on the tests by Hvorslev are proposed. Finally, the formula of shear strength with the same structure in a real sense for over-consolidated soil, normally consolidated soil, and under-consolidated soil is deduced by solving the limit and derivation according to the Hvorslev's true strength theory for over-consolidated soil.