Abstract: To solve the mutual coupling of the airborne microstrip active array antenna, a decoupling method based on characteristic mode analysis is proposed. In this method, the excited characteristic modes of the element are studied to distinguish between working and non-working modes firstly. Then, non-working modes contributing more to mutual coupling are obtained by studying the structure of the array, modal weighting coefficients, and modal characteristic far fields. Finally, non-working modes are suppressed while reducing the impact on working modes, thereby reducing the coupling while ensuring antenna performance. A microstrip antenna element of the band of 2. 19 GHz to 2. 32 GHz is designed. Non-working modes contributing more to mutual coupling are suppressed by shaping the patch of the element and adding metal columns in the element. Binary arrays before and after the improvement are compared. It can be seen that S12 in the entire band is reduced by more than 3 dB with acceptable gain drop and improvement of cross polarization. This method provides a new idea for effective decoupling of the airborne large-scale microstrip active array antenna.