In the process of production and service of aircraft, various defects in the surface and interior of composite material will occur. Thus a composite material defect detection system based on the coplanar electrode array(4×4) is designed. This system takes FPGA as the core, using the micro-capacitance measurement method of AC excitation to improve the anti-stray capacitance. Due to the poor real-time performance of the traditional simulator demodulation system, this system adopts orthogonal sequence demodulation to deal with the signal to improve the measuring accuracy. The experimental results show that the system can accurately judge the location of simulated defects, and the reconstructed image is clear and the edge artifact is less. It can provide some reference for the application of the coplanar multielectrode array sensor in the field of composite defect detection.