The invention provides a composite microbial inoculant for controlling plant continuous-cropping diseases, which comprises trichoderma harzianum, bacillus subtilis, and gliocladium roseum, and also comprises phosphate-solubilizing bacteria and potassium-solubilizing bacteria; the accession number of the trichoderma harzianum strain is CGMCC NO. 7861; the accession number of the bacillus subtilis strain is CGMCC NO. 7850; the accession number of the gliocladium roseum strain is CGMCC NO. 9008. The invention also provides a preparation method and an application of the composite microbial inoculant. Through the selection of high-efficient bacterial strains such as disease-preventing trichoderma, bacillus subtilis, gliocladium roseum, phosphate-solubilizing bacteria and potassium-solubilizing bacteria, the composite microbial inoculant of the invention promotes the growth of crops, improves the utilization rate of fertilizers, enhances the stress resistance such as disease resistance and the like, improves the quality of crops, and can control diseases such as root rot, verticillium wilt, damping off, botrytis, epidemic diseases, powdery mildew, sclerotinia rot, seedling-stage damping off, and the like.