The invention discloses a method for ex-situ preparation of methane and combined production of humic acid, and a composite microbial agent used in the same, belonging to the field of recycling of brown coal. The composite microbial agent is composed of fermentation broth of four bacteria, i.e., Escherichia coli, Pseudomonas putida, Penicillium decumbens and Clostridium acetobutylicum. The composite microbial agent has strong degradation capability on a variety of celluloses, hydrocarbons, asphaltene and a variety of carbonaceous macro-molecules. As coal is treated by the composite microbial agent, anaerobic digestion time can be shortened, output of methane gas is increased and extraction efficiency of humic acid is improved; moreover, the synergistic effect of the above-mentioned four microbial strains is superior to the effect of any one microbial strain or any two microbial strains.