Space telescopes are lightweight and have low absolute stiffness. Hence, deformations occur during ground-based testing under gravity. Once in orbit, with the release of gravity, the adjusted structures experience rebound, which significantly affects their imaging quality. Therefore, gravity unloading is essential during testing. This study investigates the deformation trends of off-axis telescopes under gravity, analyzes various unloading methods, and determines the unloading force via the position closed-loop method. The layout of unloading points is optimized based on local stress conditions, and a small-scale, easily adjustable constant-force universal unloading device is designed without interfaces. After unloading, the maximum displacement difference between the primary and secondary mirrors decreases from 44.2 μm to 4.6 μm, constituting a reduction of 90% compared with that before unloading. The maximum angular difference decreases from 9.5'' to 2.2'', constituting a reduction of 77% compared with that before unloading. The root mean square value of the wavefront aberration of the primary and secondary mirrors decreases from 0.185λ to 0.0292λ, constituting a reduction of 84% compared with that before unloading. The contact surface between the device and telescope is optimized by reducing the local maximum pressure from 49.8 kPa to 30.0 kPa, constituting a decrease of 40%. This unloading method satisfies the requirement of maintaining the relative position stability between the primary and secondary mirrors before and after gravity release.