Abstract: Saturated shear strength is a key soil property and the reference of unsaturated shear strength, however, the variation of which is obvious. The mechanism of the variation of saturated strength was investigated. The direct shear tests under consolidated drained condition after the samples swelling to steady state were carried out for Jingmen expansive soil associated with six levels of compactness. The direct shear test results were compared with the results of triaxial compression tests. Comparative investigation finally discovered that the causation of the variation was the different void ratios under the same stress state, and the difference of void ratios was resulted from pre-stress history and hydration state. Under the hydration state of the swelling was complete, the effective cohesion decreases with the increasing void ratio regularly, the effective angle of internal friction was insensitive to void ratio which can be considered as a stable value. Under the hydration state of the swelling was restricted by the saturator (void ratio is relatively lower) which is the conventional saturation method, the effective angle of internal friction is significantly higher than that of the previous hydration state. That was because the swell potential increases with the decreasing void ratio, a higher shear resistance can be mobilized by the dialtancy induced by higher swell potential. Since the effective angle of internal friction under the hydration state of the  complete swelling is low and stable, it was suggested that the saturated strength parameters under that state should be adopted.