This invention discloses a method for preparing Rhizobium inoculation agent by using city sludge. The method comprises: adjusting the sludge culture medium, culturing by shaking, fermenting in a seed tank, fermenting in a fermentation tank, post-treating the fermentation liquid and examining the product quality. The method can not only treat the sludge, but also obtain Rhizobium inoculation agent product with high added values, thus lowering the cost of sludge treatment. The obtained Rhizobium inoculation agent product is liquid or solid, and can effectively accelerate nodulation and nitrogen fixation of leguminous plants. The Rhizobium inoculation agent product can provide nitrogen nutrition for leguminous plants, reduce the application of nitrogen fertilizer, improve the physicochemical properties of the soil, and improve the yield and quality of leguminous plants.