[Objective] The effects of vermicompost co-applied with inorganic fertilizer were measured in a watermelon field to provide some references for soil improvement and watermelon plantation. [Methods] Five treatments with different mixture ratios of nitrogen(supplied from urea) and vermicomppost were set in a field experiment they were CK(neither urea nor vermicompost was applied), N₁₀₀(100% of nitrogen), M₁₀N₉₀(10% vermicompos and 90% of nitrogen), M₃₀N₇₀(30% vermicompost and 70% nitrogen), and M₅₀N₅₀(50% vermicompost and 50% nitrogen). Soil labile organic carbon, carbon pool management index(CPMI), enzyme activity and yield of watermelon were measured. [Results] The contents of highly labile organic carbon, mid-labile organic carbon and labile organic carbon significantly increased under M₁₀N₉₀, M₃₀N₇₀ and M₅₀N₅₀ treatments in comparison with N₁₀₀ treatment. The labile organic carbon content and CPMI in M₃₀N₇₀ treatment were obviously higher than that of other treatments, increased 30.10% and 37.28% as compared with N₁₀₀ treatment, respectively. At the same time, the M₃₀N₇₀ treatment markedly increased in urease and invertase activities in comparison with other treatments, showing 84.66%, 62.33%, 47.26% and 22.46% increments in invertase activities over the values of CK, N₁₀₀, M₁₀N₉₀ and M₅₀N₅₀, respectively. In addition, the yield and fertilizer productivity of watermelon also achieved the highest value in M₃₀N₇₀ treatment and had significant differences with other treatments, obtained 26.49%, 13.34%, 6.27% and 166.50%, 54.11%, 21.37% increments as compared with the treatments of N₁₀₀, M₁₀N₉₀ and M₅₀N₅₀, respectively. Correlation analysis revealed that soil labile organic carbon and carbon pool management index may be better indicators than soil total organic carbon in reflecting the changes of soil enzyme activities, yield and fertilizer productivity of watermelon. [Conclusion] The application of vermicompost co-applied with inorganic fertilizer, especially the M₃₀N₇₀ treatment, had remarkable promotion effect in watermelon plantation.