Parallel plate capacitor is a core mechanism in a Micro-mechanical-electrical (MEMS) sensing device. As the surface roughness of an electrode has obviously impact on the space electric field when the distance between the electrodes is shortened in the capacitance detection, this paper explores the effect of the surface roughness of electrode on the performance of the parallel plate capacitor. A parallel plate capacitor model with a single roughness electrode was established, and then the finite element method was used to study the effect of the surface roughness on the detecting capacitance. Based on increasing rough surface to enhance the memory electric charge capability, the formula for parallel plate capacitor with rough surface was corrcted. Finally, the Atomic Force Microscopy(AFM) was used to describe the samples with different surface roughnesses. Experiments and simulation results indicate that the surface roughness has a obviously effect on the detecting capacitance. Increasing the surface roughness of an electrode and decreasing the distance between the electrodes can improve the detecting capacitance greatly. When the surface roughness of an electrode increases from 0.063 nm to 60 nm, the detecting capacitance value grows by 9.0 percent. The result shows that increasing the surface roughnesses of electrodes can improve the sensitivity of MEMS devices.