Abstract: The microbial inoculants used in agriculture have been proved to have good effects on plant growth without enviromental pollution. Therefore they have been developed and extensively used in agriculture as biofertilizers, biocontrollers and agents for decomposing chemicals remained on food crops or in soils. But the less understanding of the mechanism on plant growth obstacles in a bit the development and proper use of microbial inoculants. Thus there is an urgent need to carry out related researches on this aspect. The present paper mainly summarized the reasearch progresses on the mechanism of micraobial inoculants on plant growth promotion.