In the background of promoting common prosperity, the extent of the real income disparity is a key element of research on regional economic development. To address the problem that nominal income cannot reflect the real living standard and welfare level, the paper uses subnational Purchasing Power Parity (PPP) as an entry point, construct a generalized framework of the multilateral comparative index system under the stochastic approach by improving the existing stochastic approach, and uses the improved method to accurately measure the subnational PPPs of 31 provinces in China, thereby quantifying the real income disparity. The results show that compared with nominal income, the real income increased significantly, and the real income disparity between regions narrowed, with the ratio of the highest to the lowest real income in China decreasing from 2.39 to 2.03. Real income excluding the effect of prices showed a new characteristic of moving towards the north for the high-income areas and towards the south for the low-income areas. The results of the study provide a more objective and accurate index of real income and provide data support and reference for differential policy formulation among regions in China, thus promoting the realization of common prosperity.