A decoupled sliding mode control strategy based on finite-time sliding mode disturbance observer and double power combination function reaching law was proposed for the position control of a spherical rolling robot in longitudinal motion. The proposed control strategy can not only guarantee the rolling performance of the spherical robot, but also can keep the continuity of the decoupling sliding mode control law. A finite-time sliding mode disturbance observer was designed to estimate the system uncertainties on-line, and the estimation of the balance pendulum angle was obtained. Based on the estimation of the balance pendulum angle, the sliding mode variables of both levels were defined respectively. Using the designed finite-time sliding mode disturbance observers, the decoupled sliding mode control law was constructed based on the second level sliding mode variable and double power combination function reaching law. It is proved theoretically that the proposed control strategy can ensure the convergence of the sliding mode variables of both levels to 0. The simulation results verify the controllability and robustness of the proposed control strategy.