[Objective] The changes of soil organic carbon(SOC), soil microbial biomass carbon(SMBC), soil microbial biomass nitrogen(SMBN) and carbon pool management index(CPMI) under different amount of topdressing urea in multi-cropping rape after wheat planted were explored to provide scientific basis for urea fertilization.[Methods] Plot test was conducted in Gulang County, with five treatment levels of topdressing urea 0, 90, 120, 150, 180 kg/hm².[Results] The urea fertilizer was observed that all levels increased biological production of multi-cropping rape after wheat planted. With the increases of urea, SOC, SMBC, SMBN and CPMI all decreased firstly and increased later. CPI and CPMI changed similarly. SOC and SMBC had significant correlations with CPMI; SOC and CPMI correlated extreme significantly; SMBC and SOC correlated positively but insignificantly.[Conclusion] If biological yield of rapeseed and soil microbial biomass were both considered, the best fertilizer rate for multi-cropping rape after wheat planted is 120~150 kg/hm² in Gulang County.