There has been an increasing interest in a kind of underactuated mechanical systems, mobile wheeled inverted pendulum (MWIP) models, which are widely used in the field of autonomous robotics and intelligent vehicles. A novel structure including an MWIP system and a movable seat called UW-Car is proposed in the study. The dynamic model of UW-Car system running in the flat ground is obtained by applying Lagrange's motion equation. A sliding mode control (SMC) method is proposed for the dynamic model, which is capable of both handling the mismatched perturbation and keeping the body upright. An optimal braking scheme is introduced which reduces the velocity of UW-Car to zero first and adjusts the displacement of seat to the centre position. Genetic Algorithm (GA) is adopted to search the optimal parameters for sliding mode controller. The optimal braking scheme is implemented by on-line switching three sliding mode controllers. The effectiveness of the proposed methods is finally confirmed by numerical simulation.