An improved method, namely, combined UTD with MoM, is studied to calculate the disturbed pattern of array antennas mounted around an electrically large platform. Each array unit is fed in turns in an MoM processor and then substituted into the UTD process to pursue the calculation of the disturbed pattern. When solving the current coefficient in MoM, the linear superposition principle is used to realize the separate feeding of each array unit. The proposed method can avoid unreliable UTD results when an equivalent phase center of the antenna is sheltered, and it can improve the RMS error from more than 15 dB to about 1 dB in the T-shaped example given. No large amount of computing resources are needed for the proposed method; the accuracy, efficiency and necessity are illustrated by examples given.