Soil nutrient deficiency is a key factor limiting crop growth and yield, but also threatens the survival of crops. In order to improve soil fertility and crop yield of sloping farmland in karst area of Southwestern China, the experiments were carried out in Qianxi county, Guizhou province for two consecutive years from 2018 to 2019. We explored the contents and the ecological stoichiometry characteristics of soil carbon, nitrogen, phosphorus, potassium and revealed the soil nutrient status and soil improvement effect under different straw mulching application rates through this experiment. There were six straw mulching gradients of maize monoculture+straw mulching (SM₀-SM₅, 0, 1111, 2222, 3889, 5556, 6944 kg/hm²), in which SM₀ was the control cheak (CK). The results showed that:(1) straw mulching increased the contents of soil organic carbon (SOC), total nitrogen (TN), and total phosphorus (TP) in different degrees, and generally increased with the increase of mulching rate. Especially in SM₄ and SM₅, which were significantly higher than those of the CK (P<0.05), but there was no significant difference between them (P > 0.05), while total potassium (TK) decreased with the increase of mulching rate. (2) Under the condition of high straw mulching (SM₄, SM₅), the stoichiometry ratio of soil nutrient elements in two years was significantly higher than that of the CK (P<0.05), except for N:P, but there was no significant difference between SM₄ and SM₅ (P>0.05). (3) Under the condition of straw mulching, the C:N and C:P in soil were lower, which accelerated the mineralization and decomposition of soil organic matter, and also showed that the availability of P was higher. (4) The crop yield of each mulching treatment was significantly higher than that of the CK (P<0.05), especially under the condition of high straw mulching, the yield of crop increased significantly, and the yield of SM₄ was the highest. The research showed that straw mulch on the ground is an important source of soil nutrients, which can be used as a supplementary means to supplement the soil nutrients absorbed by crops and lost with soil erosion. Based on the crop yield, the principal component analysis, grey correlation analysis, difference significance, it is suggested that straw mulching in about 6000 kg/hm² should be adopted for sloping farmland in the karst area of Southwestern China to promote the restoration of farmland ecosystem and achieve better effect of soil fertility and crop yield.