It is found that great differences exist in the consolidated quick direct shear strength index of the silty clay based on several consolidated quick direct shear tests with different rate of shear. In this letter, the changes of the water-content coefficients of the soil samples before and after the tests are analyzed. The results indicate that the drain consolidation phenomenon exists during the consolidated quick direct shear tests. Different rates of shear are corresponding to different levels of drain consolidation, and then the strength index is different. Furthermore, the reason of the fact that the friction angle of the silty clay relatively increases with different rates of shear, while the cohesive strength decreases relatively is explained.