Taking the undisturbed loess as the research object in a certain area of Xining City, they were designed the three-way slow freezing test under the closed system, the constant temperature and moisture thawing test and the triaxial shear test. Based on the Mohr-Coulomb strength theory of soil, the cohesion and internal friction angle were solved by the shear-strength envelope. Study the variation law of its shear strength index with different freeze-thaw temperature gradients. The results show that the cohesion decay rate is significant as the temperature gradient increases, so the most unfavorable freeze-thaw temperature gradient is -15 °C~15 °C when the cohesion is minimum. It can be seen that cohesion is related to the cold end temperature and thawing temperature about the freeze-thawed loess. when the lower moisture content is under the action of the smaller cold end temperature; or the higher moisture content is under the action of the larger cold end temperature, the thawing temperature is more dominant than the cold end temperature. With the increase of the cold end temperature, the most unfavorable moisture contents is 18.34%, because the decreasing extent of cohesion of sample first decreases and then increases with different moisture contents,so the most unfavorable moisture content is 18.34%. The internal friction angle shows irregular fluctuations, the variation extent is about 0° ~ 14°. It’s fitted through the test data for the caculating formula of the cohesion between the moisture content and the freezing temperature.And it has been verified by test that can describe its variation characteristic good.