The solar magnetic field is central to understanding solar activity, making high-precision spectropolarimetric observations of great importance. Xihe-2, positioned at the Sun–Earth L5 point, is designed to observe the solar polarized spectrum. Its onboard Spectral Magnetic Telescope is expected to employ a three-retarder stepwise modulation scheme to retrieve solar magnetic field information. However, this scheme has not yet been sufficiently studied in terms of contingency operation and accuracy control. In this work, we present a systematic analysis and improvement of the scheme from both perspectives. In the event that one retarder becomes inoperative owing to stepper motor failure, the remaining two retarders must be adjusted so that all four Stokes parameters can still be measured. We examine the feasibility of such backup modulation schemes for different retarder-failure cases and derive the corresponding conclusions. We further analyze the three major error sources in the ideal three-retarder stepwise modulation scheme, determine their acceptable ranges, and perform comprehensive validation and optimization through Monte Carlo simulations. The results show that within the derived angular-error tolerances, the measurement accuracy of the retrieved Stokes parameters can reach the order of 10−3.