A complex microbial preparation comprises (in vol. %): Photosynthetic bacterium (5-15%), Bacillus (10-20%), Yeasts (20-35%), Lactobacillus (30-45%) and Actinomycetes (3-10%). The preparative method of the complex microbial preparation involves first and second level complex fermentations. The first level complex fermentation involves: (a) separately inoculating the above-mentioned bacteria on first level culture media and culturing to obtain first level bacteria, (b) separately inoculating the obtained first level bacteria on second level culture media and culturing to obtain fermenting bacterial liquids, (c) mixing the five obtained fermenting bacterial liquids to obtain first level compound bacteria. The second level complex fermentation involves: adding sterile water, Lactobacillus plantarum, Lactobacillus acidophilus, Saccharomyces cerevisiae and brown sugar to the obtained first level compound bacteria, mixing uniformly in a same fermentor, and culturing to obtain second level complex microbial preparation. The complex microbial preparation is used in treatment for diabetes.
