In terms of the instability problems in later-stage surcharge of embankment engineering, study on the effects of drainage conditions on clay strength is carried out with constant-strain-increment-ratio triaxial tests of reconstituted saturated kaolinite. Test results show that the deviator stress increases with the increase of strain increment ratio, while the excess pore pressure decreases with the increase of strain increment ratio. In the mid and later stages of tests under partial drainage conditions, the shearing samples are close to or in the critical state, while limiting the drainage or accelerating the loading velocity may cause failure.