Soil carbon (C), nitrogen (N), and phosphorus (P) conditions reflect soil fertility, and their stoichiometric characteristics play a vital role in indicating soil nutrient balance, and achieving sustainable forest management. Soil C, N and P stoichiometry are significantly altered by forest succession, however, the results are not consistent. The knowledge on the dynamics of soil C, N, P and their stoichiometry along subalpine forest succession in western Sichuan is still lacking. To explore the effects of subalpine forest succession on C, N, and P and their stoichiometry, and to reveal the mechanisms of nutrient accumulation and cycling in soils during forest succession, thereby providing a theoretical basis for evaluating and enhancing subalpine forest ecosystem service functions, four forests along a successional sequence—pre-successional broadleaf forests (Stage I), mid-successional mixed coniferous and broadleaf forests (Stages II and III), and late-successional dark coniferous forests (Stage IV)—were selected as study objects using the “space for time substitution” approach in subalpine region of western Sichuan. The changes in soil C, N, and P contents and their stoichiometry ratios in different soil layers along the succession sequence were studied, and the successional patterns of soil C, N, and P density and the key influencing factors were analyzed. The results showed that the succession stage and soil layer had a significant effect on the contents of soil organic carbon (SOC), total nitrogen (TN), and total phosphorus (TP), as well as the C:P, and the N:P, but had no significant effect on the C:N. The SOC, TN and TP contents, C:P, and N:P showed an overall trend of first increasing and then decreasing along the succession sequence, while decreasing with soil depth. The SOC, TN, and TP densities within the 0—60 cm soil layer across the different successional stages ranged from 7.27 to 15.86 t/hm2, 0.41 to 0.99 t/hm2, and 0.21 to 0.41 t/hm2, respectively, with the higest values occurring at stage II. The Mantel test results showed that the SOC density across different successional stages was significantly affected by N:P, C:P, Rao's quadratic entropy index, functional dispersion index (FDis), and available phosphorus. In addition to the above factors, TN density was also significantly affected by available nitrogen and plant richness, whereas TP density was only significantly correlated with C:N and available phosphorus. The results revealed the effects of forest succession on soil C, N, and P contents, density, and stoichiometry, emphasizing the important role of relative soil nutrient limitation and plant functional diversity in regulating the accumulation of C, N, and P along the succession sequence. This can provide scientific basis for the improvement of soil function in subalpine areas of western Sichuan.