The invention discloses composite microbial inoculum and an application thereof to compost fermentation of alpinia speciosa K.schum. branches and leaves. Active components of the composite microbial inoculum comprise the following raw materials in percentage by weight of 26%-34% of haloplasma contractile, 13%-17% of pusillimonas soli, 13%-17% of lutispora thermophila, 13%-17% of clostridium butyricum, 8%-12% of methylococcus capsulatus, 4%-6% of tepidanaerobacter acetatoxydans, 4%-6% of proteiniborus ethanoligenes and 4%-6% of ruminococcus flavefaciens. According to the composite microbial inoculum, floras are in mutualism and synergy and free from an antagonism phenomenon. The alpinia speciosa K.Schum. branches and leaves fermented with the microbial inoculum are used for preparing a biological organic fertilizer, so that the compost megathermal period ( greater than or equal to 50 DEG C) can be advanced by 8-10 days, and prolonged for 3-5 days, and the biological organic fertilizer can become thoroughly decomposed rapidly.