[Objective] To address the challenge that controllable excitation linear synchronous motor (CELSM) faces in simultaneously achieving high-precision trajectory tracking, strong disturbance rejection, and smooth control input under complex sudden load disturbances, an improved variable-gain super-twisting sliding mode control (IVG-STSMC) strategy is proposed. This strategy aims to enhance the system’s dynamic response quality and suppress the high-frequency chattering inherent in conventional sliding mode control. [Methods] Firstly, a mathematical model of the CELSM in the d-q coordinate system was established. On this basis, a continuous Softsign function was employed to replace the traditional sign function, and a variable-gain adaptive adjustment mechanism based on state errors was introduced to construct the VG-STSMC controller. This step was taken to alleviate the conflict between chattering and convergence speed caused by the fixed gain and discontinuous switching of the sign function. Furthermore, a nonlinear disturbance observer (NDOB) without acceleration measurement was incorporated into the VG-STSMC to estimate the lumped disturbance online. Finally, a composite IVG-STSMC control framework was established through feedforward compensation. [Results] Simulation results demonstrated that under the condition of sinusoidal reference trajectory tracking with an amplitude of 1 mm and a frequency of 0.5 Hz, IVG-STSMC outperformed both proportional integral control and conventional SMC overall in terms of tracking accuracy, speed smoothness, and current chattering suppression. Under the compound working condition where a 50 N step load was applied at t=1.0 s and removed at t=2.5 s, a 51.8% reduction in maximum tracking error was achieved by IVG-STSMC compared to VG-STSMC, and a 65.5% reduction in peak error was recorded at the exact moment the disturbance was introduced. Furthermore, the smoothness of the speed and q-axis current responses during disturbance switching was further improved by IVG-STSMC. [Conclusion] The proposed strategy effectively coordinates trajectory tracking accuracy, disturbance rejection capability, and control input smoothness of the CELSM feed system under complex load disturbances, verifying the effectiveness of the synergistic mechanism between variable-gain higher-order sliding mode regulation and disturbance feedforward compensation.