The aim of this study is to reveal the roles of MnO2 in Maillard reaction of biotic composting, and to identify its effectiveness in promoting humus formation. Corn straw (CS) and chicken manure (CM) have been chosen to be composted. During CS composting, addition of MnO2 rapidly reduced reducing sugars concentration (decreased by 84.0%) in 5 days and significantly improved humus production by 38.7% compared with treatment without MnO2. Whereas in CM composting, the promoting effect of MnO2 on humus formation was relatively weak by comparing with the treatment group of CS. Additionally, the presence of MnO2 has reshaped bacteria community, which might be the reason of MnO2 stimulated bacteria to utilize organic matter during CM composting. Therefore, the structural equation modeling has confirmed that MnO2 mainly performed as chemical catalyst to promote humus formation during CS composting. Besides catalyst, MnO2 also played as a bioactive activator in CM composting.