The planar capacitance tomography (ECT) technique is usually used to reconstruct the image of bonding defects of composite materials by means of sensitivity matrix. The sensitive field of ECT sensor is distributed in a three-dimensional space, but the reconstructed image for defect detection is two-dimensional, so there is the problem of selecting the sensitivity matrix of different layers. The selection of sensitivity matrix in different layers is explored, and an improved method of sensitivity matrix is proposed. Through the threshold processing and absolute value processing of the negative sensitive region, the ill condition and non-uniformity of the sensitivity matrix are improved, and the modeling error of ECT forward problem is reduced. Simulation and experimental results show that the proposed method can significantly reduce the artifacts in the reconstructed image, improve the clarity of the reconstructed defects, and then reduce the probability of false or missed judgment.