Abstract: Based on the researches of the other scholars and the critical state soil mechanics,theoretical formulas of the undrained strength under K0 compression and extension stress condition were derived with consideration of induced anisotropy.After collecting contrasting the existing test results,it was found that the results abtained with theoretical formulas proposed in this paper agreed with the test results and back analysis of failured dams.Based on a large number of investigations,a simple formula for calculating the mean undrained strength was proposed,and it the method could be popularized for engineering application.