The invention relates to application of straw decomposition and low-temperature methane production dual-function composite microbial inoculum, which is used in a rural household methane tank which takes crop straws as main raw materials or in a household methane tank which has been started in winter in the northern area and takes feces or the crop straws as main raw materials, so as to promote the fermentation of the straw raw materials and guarantee normal methane production of the methane tank under the condition of low temperature in winter. The methane production effect of the rural household methane tank which adopts the method is good; after addition of the composite microbial inoculum into the methane tank which takes the straws as the main raw materials under normal temperature, the starting time is shortened by 5 to 12 days compared with a contrast tank and the average methane production amount per day is improved by 30 to 50 percent compared with the contrast tank; and the normal use of the methane tank can still be guaranteed under the condition of the lower temperature in winter. The invention is mainly suitable for the rural household methane tank, has the advantages of quick effect, long effect, low cost, simple use method, easy popularization and so on, and can be promoted and used.