To study the yield and critical state of over-consolidated saturated clay, a series of undrained shear tests of saturated clay in different over-consolidated states were carried out by GDS triaxial tester. To analyze the stress path of saturated clay,and the ALPHA(α) model was selected to describe the yield characteristics of soils in different over-consolidation states. The variation trend of deviatoric stress/pore pressure - strain curves of saturated clay was analyzed , The failure state was identified when the pore pressure increased to the maximum value. The critical state line is fitted according to the stress failure points. The calculated formulas for the undrained shear strength with different over-consolidation ratios are presented. The results show that with the same preconsolidation pressure, the shear strength increases with the decrease of over-consolidation ratio. However, the yield stress do not monotonically increases with the decrease of over-consolidation ratio. With the same over-consolidation ratio, the shear strength increases with the increase of the preconsolidation pressure, and so dose the yield stress. The failure strain of normal consolidated soil and light over-consolidated soil is about 5%, while that of heavy over-consolidated soil is less than 2.5% with obvious dilatation and brittle failure. The stress paths of normal consolidated and light over-consolidated soils are "S" type, while the stress paths of heavy over-consolidated soils are in trend of increasing. According to the failure criterion, heavy over-consolidated soils share the same critical state line with light over-consolidated and normal consolidated soils. There are few formula parameters for calculating undrained shear strength, which is easy to be obtained and embedded in the constitutive models, and it is convenient for application.