To explore effect of biological soil crusts (BSCs) on soil microbial biomass carbon（SMBC）and nitrogen（SMBN）, soil of a sand dune covered with BSCs in the artificially revegetated sand zone at the southeastern edge of the Tengger Desert, China was selected as subject of the study. Four sample plots were set different in sand-fixing time (55, 47, 30 and 20 a, separately), and a plot of mobile sand dunes and a plot of natural vegetation as control. Results show that cyanobacteria-lichen and moss crusts significantly increased the contents of SMBC and SMBN in artificially revegetated areas (p<0.05), which was positively related to sand-fixing time（p<0.05). Crust type was also a factor, significantly affecting the content of soil microbial biomass. The contents of SMBC and SMBN under moss crusts were significantly higher than those under cyanobacteria-lichen crusts（p<0.05). In addition, BSCs significantly increased the contents of SMBC and SMBN in the 0~20 cm soil layers（p<0.05）and the effect weakened with increasing soil depth. Moreover, the contents of SMBC and SMBN under BSCs varied seasonally to a significant level, showing a decreasing order of in summer > in spring > in autumn. Heat and water were two main factors controlling the seasonal variation, while BSCs affected soil microbial biomass seasonally by regulating soil temperature and moisture.