A compact encoder-based focusing mechanism is proposed to compensate for axial focal-plane displacement in a space optical camera. The mechanism converts the rotation of a stepper motor into linear motion of the CCD focal plane assembly through a worm gear pair, a gear pair, and a cylindrical cam pair, while an encoder provides position feedback for closed-loop control. A displacement-count transmission model was developed and verified through MTF simulation, positioning tests, autofocusing experiments, finite element analysis, and vibration tests. The mechanism achieved a single-step control resolution of 1.38 μm and an encoder-based displacement measurement accuracy of 0.486 μm. The positioning test showed a maximum absolute error of 7 μm within a ±2 mm travel range, satisfying the ±7.5 μm requirement. Optical simulation indicated that axial defocus broadened the PSF and reduced the MTF near the 50 lp/mm Nyquist frequency. Autofocusing experiments using a resolution target showed that the normalized stripe contrast recovered to its maximum near the best-focus position, supporting the practical image-quality recovery trend predicted by the MTF simulation. The measured first natural frequency was 174.64 Hz, and the random vibration amplification ratios were below 3 in all tested directions. The results demonstrate that the proposed mechanism can provide accurate and mechanically robust compensation of focal-plane displacement.