The invention belongs to the technical field of micro-organisms and soil fertilizers. Straw return to fields is a universally concerned agronomic measure for increasing soil fertility and increasing yields and efficiency in the world, both to achieve pollution-free processing of straws, but also to increase the soil fertility. However, if the decomposition of straws in the soil lasts for a long time, large soil void will be caused, to result production problems such as dead seedlings, and freeze injury are prone to generate, and to enable long-term survival of pests in the soil, bringing potential harm to agricultural production. The present invention provides an optimized preparation process for a fermentation broth by using maize straws as a main raw material, and Penicillium oxalicum (Accession No. CGMCCNo.4842) as a seed bacterium; and at the same time provides application methods of the fermentation broth in agricultural production. The fermentation broth can be used to accelerate decomposition of maize straws, and is suitable for straw return to field areas to promote maize straws decomposition and increase the soil fertility.