We designed and fabricated an electrostatically actuated micro-electro-mechanical system (MEMS) optical phase shifter incorporating a comb capacitive displacement sensor. To overcome the technical challenges of high driving voltage and inadequate control precision in current MEMS optical phase shifters, this study employed a hollow serpentine elastic beam structure to reduce the out-of-plane vertical stiffness of the elastic beam within the constrained chip dimensions, thereby lowering the actuation voltage. Furthermore, a comb-capacitive displacement sensor was integrated to facilitate an optical mirror displacement sensing system, significantly enhancing control accuracy. Experimental results indicate that at a driving voltage of 59 V, the micromirror achieves an optical phase modulation of 2.139 5π rad. The integrated comb-capacitive sensor demonstrates a detection error of less than 0.5 fF, corresponding to an optical phase-shifting error below 0.016 2π rad. This device shows promising potential for applications in optical communication systems and vector optical phased arrays as a phase modulation element.