To address the problem of insufficient preload in screw-rod linear ultrasonic motors（LUMs），we developed a novel screw-rod LUM with an adjustable preload by incorporating a separable stator structure capable of bidirectional preload adjustment. Through theoretical analysis and experimental research，a preload-friction model was established to reveal the relationships among the preload，output torque，and critical speed. The corresponding preload-output torque–speed curves were derived. A prototype of the ultrasonic motor was developed，and an experimental platform was constructed. The relationship between the output force and preload was fitted to a quadratic function. The experimental results demonstrated that the optimized novel structure improved the reverse output force by 20.3%，thereby providing theoretical and practical support for the application of screw-rod ultrasonic motors in high-precision drive fields.