In order to characterize the defects of carbon fiber reinforced plastics (CFRP) by using the capacitance directly measured by planar capacitance sensor, a planar capacitance sensor is constructed in COMSOL based on the capacitance edge effect, and the influence of electrode parameters on its performance was analyzed and optimized, The sensitivity has been improved by 2% ~ 52%, and the penetration depth can reach 4.13mm. When the sensor area is constant, we can find the optimal length to width ratio of the sensing electrode, and then we can fit the linear relationship between the electrode length and area. The change in the capacitance value of the sensing electrode is detected when the defect size is varied with a single electrode DC excitation. The results show that the capacitance value of the sensing electrode decreases as the defect size increases, and the quantitative evaluation of CFRP defects can be achieved using the fitted curve with a correlation coefficient of 0.996 or more.