The invention discloses a combined anaerobic fermentation method of organic solid wastes. The organic solid wastes used for the combined anaerobic fermentation include domestic wastes, excess sludge, feces, kitchen waste, straws, etc. The method comprises the following steps: carrying out different pre-processing processes to obtain organic materials with a granularity less than 5 mm; passing through a cutting pump, adjusting the C/N ratio, adjusting the water content, etc. to obtain a homogeneous fermentation substrate with a solid holdup of 2 to 10%; hydrolyzing and acidifying the fermentation substrate in the presence of a hydrolase; fermenting for a period of 15 to 25 days under the condition of stirring at a middle temperature of 32-38 DEG C to obtain biogas, which can be used for energy supply or output of a system; aging and desalting the biogas liquid to obtain a liquid humic acid fertilizer; and processing the biogas residues to a granular humic acid fertilizer. The fermentation substrate has proper C/N ratio to obviate feedback suppression of the substrate during the fermentation of a single material and simultaneously can enhance the hydrolysis effects of celluloses, lignin, hemicelluloses, etc. The method has the advantages of easy flow pattern control, low energy consumption, and no generation of sewages, and can obtain the high-quality biogas fluid and the high-quality granular humic acid fertilizer.