To address the degradation of trajectory tracking performance of quadrotor unmanned aerial vehicles (UAVs) under external disturbances, a sliding mode trajectory tracking control method based on disturbance observation is proposed in this paper. A unified dynamic model for the translational and attitude motions of the UAV is established, where external disturbances are treated as unknown equivalent inputs. Then, a sliding mode tracking controller is designed, and a disturbance observer is introduced to estimate external disturbances online. The estimated disturbances are compensated in a feedforward manner to enhance the disturbance rejection capability and tracking accuracy of the system. The proposed method does not rely on an accurate disturbance model and features a simple structure and strong robustness. Simulation studies based on spiral trajectory tracking are conducted to compare the performance of the sliding mode controller and the sliding mode controller with disturbance observation. The results demonstrate that the proposed method effectively suppresses the influence of external disturbances and significantly improves the position and attitude tracking performance of the quadrotor UAV, validating the effectiveness and feasibility of the proposed control strategy.