A composite microbial system SYF with efficient cellulose-degrading capacity was constructed from rhizosphere soil of mangrove plant rich in bio-substances by limited generation selections. It was found that 83.6% of Bruguiera gymnorrhiza leaves, 71.5% of Rhizophora stylosa leaves and 67.3% of filter paper were degraded by the composite microbial system at 30 ℃ within 10 days under static culture conditions with the CMCase activity of 102.2, 126.3 and 110.7 U/mL respectively. The microorganism diversity of SYF was analysis by the plate screening method and clone library establishing method. Eight genera of bacteria and 3 genera of fungi were isolated from the composite microbial system. Six kind of bacteria with known genera and two with unknown genera were detected by the 16S rDNA libraries, while 5 species of fungi were detected by the ITS libraries. Only 3 genera of the microorganisms were detected by both methods. .