The invention relates to a composite microbial inoculum used in a biogas generating pit, which consists of four kinds of microbial fermentation liquor, namely, 0.06-0.15 part of pseudomonas fluorescens solution, 0.04-0.08 part of waxen bacillus solution, 0.06-0.10 part of bacillus subtillis solution and 0.05-0.08 part of bacillus circulans solution, which are mixed in proportion. The composite microbial inoculum can promote the biogas generating pit to generate biogas normally in winter and can prepare the biogas by taking straws as main raw material.