The application provides a preparation method and use of a diatom-bacterium symbiotic organic liquid fertilizer, and belongs to the technical field of biological fertilizers. The preparation method of the diatom-bacterium symbiotic organic liquid fertilizer includes the following steps: (1) mixing a compound bacterial powder with a waste liquid organic matter to allow first fermentation to obtain a small-molecule organic liquid fertilizer; (2) mixing diatom with the small-molecule organic liquid fertilizer to allow second fermentation to obtain a bio-organic fertilizer; and (3) mixing a microbial inoculant with the bio-organic fertilizer to allow third fermentation to obtain the diatom-bacterium symbiotic organic liquid fertilizer. In the application, the diatom-bacterium symbiotic organic liquid fertilizer prepared by the preparation method is used for soil improvement. The organic liquid fertilizer can not only effectively improve soil, enhance fertilizer utilization, and increase crop yield and quality, but also can achieve resource utilization of waste.