Understanding the ecological stoichiometry characteristics of soil nutrient elements and their ecological responses to environmental variation are fundamental for the restoration and reconstruction of degraded forest ecosystems in the karst rocky desertification area of Southwest China. In the present study, to investigate the stoichiometry and environmental influencing factors of soil C, N, P, and K nutrient elements of karst rocky desertification ecosystems, Yachi, Hongfenghu, and Huajiang, Southwest China, were selected as experimental sites. Ninety sample plots in the three experiment sites were set to study the stoichiometry and environmental influencing factors of soil C, N, P, and K nutrient elements. The results showed that the average contents of soil C, N, P, and K in the three experimental sites were 45.61, 2.54, 0.79, and 3.33 g/kg, respectively. The average values of C:N, C:P, C:K, N:P, N:K, and P:K were 19.56, 65.07, 23.65, 3.45, 1.32, and 0.39, respectively. Among the four soil nutrient elements studied, the variation of K was remarkably higher than that of the others. There were significant (P < 0.05) differences in the soil nutrient contents or stoichiometric ratios among the different experimental sites, rocky desertification degrees, or vegetation cover rates. Contents of soil C, N, and P of no rocky desertification were significantly more than those of potential, slight, and severe degrees of rocky desertification. In contrast, among the five degrees of rocky desertification, soil K content of severe rocky desertification was the highest. There were significant nonlinear correlations among soil nutrient contents, and significant nonlinear correlations between soil nutrient contents and their stoichiometric ratios. Precipitation, temperature, percentage of exposed rocks, and land cover were the main environmental influencing factors for the stoichiometric characteristics of soil nutrients in the karst rocky desertification ecosystem. Results of the present study have important implications for the theoretical improvement of soil ecological stoichiometry and scientific rehabilitation and reconstruction of degraded forest ecosystems of karst rocky desertification in Southwest China.