[Objective] Wind-water erosion crisscross region in Loess Plateau is a typical fragile ecosystem. Biological soil crusts (BSCs) are widely distributed in this region and play key roles in the accumulation and transformation of soil organic carbon (SOC). However, the effect of BSCs succession and microorganisms on enzymes related to organic carbon conversion is still unclear.[Methods] In this manuscript, SOC contents, microbial number and enzymes related with carbon conversion in different BSCs types were investigated. The relationships among SOC, microbial number and enzume activities related with carbon conversion were also analyzed by SEM model.[Results] (1) BSCs succession significantly inceased SOC content and enzyme activities related with carbon conversion. SOC and enzyme activities in BSCs layer(0—2 cm) were significantly higher than those in bare sand 2—5 cm soil layer (p<0.05). (2) The number of bacteria, fungi and actinomyces in moss crusts was the highest, and the number of fungi and actinomyces significantly increases with BSCs succession. Bacteria and fungi in the BSCs were significantly higher than those in 2-5 cm soil layer. However, actinomycetes in BSCs (except moss crusts) were lower than 2-5 cm layer. (3) SEM model indicated that SOC, enzyme activities and microbial number showed significant correlation. [Conclusion] The accumulation of SOC by BSCs succession provided a rich substrate and nutrients for enzymes related with carbon conversion and microbes,which were helpful for the increase of microbes and improvement of enzyme activities related with carbon conversion. Microbes play critical roles in driving enzyme activities related with carbon conversion and maintaining carbon balance in BSCs.