This paper proposed an over sampling successive approximation (OSSA) technique to build switched-capacitor capacitance-to-voltage convertor (SC-CVC) for readout circuit of MEMS differential capacitive sensor. The readout circuit employing the OSSA technique has significantly improved resistance to common-mode parasitic capacitance of the input terminal of the readout circuit. In the OSSA readout circuit, there are 5 main non-ideal characteristics: holding error, recovery degradation, increment degradation, rise-edge degradation and charge injection which reduce the accuracy and the settling time of the circuit. These problems are explained in detail and their solutions are given in the paper. The OSSA readout circuit is fabricated in a commercial 0.18um BCD process. To show the improvement evidently, a reported traditional readout circuit is also reproduced and fabricated using the same process. Compared with the traditional readout circuit, the proposed readout circuit reduces the affect of common-mode parasitic capacitance on the accuracy of SC-CVC by more than 23.8 dB, reduces power dissipation by 69.3%, and reduces die area by 50%.