A field experiment was conducted to study the temporal and spatial changes of active organic C and N in the newly cultivated light sierozem soil under fava bean/maize intercropping system at Hong Shipu, in Ningxia Region. The results showed that the contents of soil microbial biomass C and N (SMBC, SMBN) and soil soluble organic C and N (SOC, SON) in this newly cultivated soil were low in comparison with long-term cultivated soil. Soil microbial biomass C increased with the addition of N fertilizer. The contents of SMBC and SMBN in soil after harvesting maize were significantly higher than harvesting fava bean. The contents of SMBC and SMBN were the highest between lines of fava bean, followed by on lines of maize, and between lines of maize. Application of N fertilizers significantly increased SOC and SON contents at fava bean harvest time in comparison to the treatment without addition of N fertilizer; and had no effect on SOC and SON contents at maize harvest time. The contents of soluble organic C and N in soil were the highest between lines of maize, followed by between lines of fava bean, and on line of fava bean, and between lines of maize and fava bean, and the lowest was in on line of maize. Significant increase of SMBC and SMBN in soil at maize harvest phase suggested that immobilization of mineral N by soil microbial community could improve the fertility of the newly cultivated soil.