Abstract: The Microwave Imager Combined Active and Passive (MICAP) is a primary payload onboard the HY-4A ocean salinity observation satellite. It integrates L-, C-, and K-band synthetic aperture microwave radiometers with an L-band microwave scatterometer, enabling high-precision measurements of sea surface salinity, sea surface temperature, and sea surface wind fields. The active and passive subsystems share a parabolic cylindrical reflector antenna, whose radiation-pattern geometric accuracy and stability are critical to synthetic aperture radiometer imaging performance. During in-orbit operation, thermally induced structural deformation can cause the in-orbit antenna pattern to deviate from that measured on the ground. These discrepancies propagate through the system response matrix into brightness temperature (BT) retrieval, leading to systematic retrieval errors. Conventional thermo–structural–electromagnetic coupled modeling requires high-fidelity finite-element analysis of the deformed reflector surface followed by electromagnetic simulations, which is computationally expensive and unsuitable for rapid end-to-end performance evaluation. To address this limitation, a rigid-body displacement equivalence algorithm is proposed to model reflector thermal deformation and is applied to the MICAP one-dimensional interferometric radiometer. The method approximates thermal deformation by applying rigid translations and rotations to the nominal reflector surface. The fit is considered acceptable when the root-mean-square (RMS) residual between the equivalent and deformed surfaces is below 0.001 times the operating wavelength. Based on the equivalent reflector model, the antenna radiation pattern is computed and used for forward radiometer simulation and subsequent BT retrieval, enabling rapid end-to-end evaluation from thermal loading to BT retrieval performance. A sensitivity analysis of two critical degrees of freedom is further conducted to quantify their impact on BT retrieval accuracy. Results demonstrate that the proposed method substantially reduces computational cost while maintaining high accuracy, providing an efficient and practical framework for on-orbit performance assessment and error compensation in spaceborne synthetic aperture radiometers.