Abstract: The radiation performance of an Active Phased Array Antenna (APAA) was known to be significantly degraded by its surface distortion. A bottleneck problem where computational efficiency and prediction accuracy were difficult to balance existed in traditional electrical performance prediction methods. To solve this problem, a fast prediction method for the electrical performance of large arrays based on the pattern decoupling of in-array elements was proposed in this paper. The element pattern was decoupled into a pointing deviation term and a mutual coupling effect term via Taylor expansion: the pointing deviation term was calculated by the central difference of an isolated element, and the mutual coupling effect term was equivalently replaced by the Active Element Pattern (AEP) extrapolation method. The experimental results indicate that compared with the pattern multiplication method, the proposed method maintains high prediction accuracy of over 90% under different surface deformation levels, and key electrical performance indicators (gain, sidelobe level, beamwidth) are highly consistent with the full-wave simulation results. Meanwhile, its computational efficiency is improved by several orders of magnitude compared with the HFSS full-wave simulation.