The invention discloses an organic biofertilizer with an additive and a preparation method thereof. The organic biofertilizer comprises the following ingredients: 40 percent of human manures, poultry manures and animal slurry, 30 percent of waste material slags and 30 percent of crop straws serving as raw materials; and one part of the additive (in an account which is 0.1 percent based on the total weight of the raw materials) is added, which comprises 50 percent of composite bacterial preparation NSC-7, 2 percent of bacillus subtilis, 2 percent of bacillus cereus, 30 percent of lactobacillus, 10 percent of polyaspartic acid, 3 percent of alpha-naphthylacetic acid and 3 percent of indolylacetic acid. The preparation method comprises the following steps of: mixing the raw materials and the additive, stacking in a cement tank, fermenting for 24 hours, adding the rest additive, stirring, and continuously stacking for 2 hours; and granulating and packaging by using the technology for granulating the organic fertilizer. The organic biofertilizer with the additive has the advantages of easy material acquirement, low cost, improvement of the soil, balanced nutrition of the soil, improvement of the quality of crops, yield increase, improvement of the gain for farmers, contribution to environmental protection, wide market prospect and positive effect on the development of enterprises and agriculture.