Moving mass control (MMC) is considered a promising approach to regulating the attitude of an aircraft via the motion of internal moving masses. The present investigation proposes a moving mass control scheme for a high-altitude long-endurance (HALE) unmanned aerial vehicle (UAV) with a single slider moving longitudinally. To that end, a longitudinal nonlinear motion model is established, and pitch dynamics are analyzed. Furthermore, the influence of the slider parameters on the dynamic characteristics of the UAV and the change in control efficiency with speed are analyzed. Finally, a detailed root-locus-based stability and sensitivity analysis of the proposed control scheme is formulated. At an altitude of 20,000 m, the MMC scheme’s efficiency coefficient was 200% of that of elevator scheme at the cruise speed. The simulation results show that the control efficiency of the moving mass control scheme was significantly higher than that of the traditional elevator control scheme under the conditions of high altitude and low speed.