The problem of sliding mode control for uncertain systems subject to input constraint is considered. There exist uncertainties in state matrix and input matrix, respectively. Moreover, the system states are unmeasured. An integral sliding surface is constructed in the space of state estimation. And then, a sliding mode controller based on state observer is designed such that the state trajectories can be driven onto the specified sliding surface in finite time despite the parameter uncertainties and input constraint. By means of equivalent control law, a sufficient condition is given to guarantee the asymptotic stability of the sliding motion. Finally, an example is provided to illustrate the effectiveness of the proposed method.