For the polarized scanning atmospheric corrector (PSAC) onboard the Huanjing Jianzai-2A/B (HJ-2A/B) satellites, this study analyzes the noise terms in the signal chain and their correlation with the signal in detail and builds a signal-total noise power linear (S-TNPL) model. Experimental measurements are conducted to obtain the coefficient of the model in which the noise power of each channel only varies with the signal and the background noise power (intercept) that is related to operating conditions rather than to the signal. An on-orbit signal-to-noise ratio (SNR) evaluation method independent of homogeneous regional scenes is developed with background noise measured on orbit and is then verified by a comparison with the onboard diffuser method. The comparison shows that the results of the on-orbit SNR evaluation of the PSAC obtained by the proposed S-TNPL model and the onboard diffuser method exhibit a relative deviation of -0.62%-6.27%, indicating favorable consistency and thus verifying the validity of the proposed on-orbit SNR evaluation method. This SNR evaluation method can not only adapt to degrading on-orbit radiation performance but also facilitate the conversion of SNRs under different radiance conditions. Moreover, it can be directly applied to evaluate the SNR of the result of any single observation. The SNR analysis method in this paper is also applicable to other optical remote sensors theoretically and can thus provide a reference for the on-orbit SNR evaluation of such remote sensors.