The invention discloses a biological prevention and control bacterial strain for preventing and controlling rice sheath blight, a biological organic fertilizer, and a preparation method of the biological organic fertilizer, which relate to biological prevention and control bacterial strains of rice and biological organic fertilizers thereof, and belong to the technology of intensive agricultural production. According to the invention, Trichodermaasperellum12, which plays an obvious role in biological prevention and control on the rice sheath blight, is obtained by separation, the biological organic fertilizer is prepared by composting the biological prevention and control bacterial strain, rice hulls, straws and poultry excrement and contains bacterial strain hypnospore of more than 0.5*10<8>cfug<-1>, the water content of the fertilizer is smaller than 15% and the organic content of the fertilizer is more than 35%. Experiments show that after the biological organic fertilizer is applied into rice fields, most of matter in the organic fertilizer can keep afloat, the hypnospore of the biological prevention and control bacterial strain, which is adsorbed on the organic fertilizer, can rapidly breed on water, aerial mycelium is formed at a water-gas junction zone, and a hypha belt is formed at the contact part of rice stalks and the water surface, so that rhizoctonia solani of the rice sheath blight on the water surface is effectively prevented from contacting the rice stalks, and the prevention and control rate of the rice sheath blight is more than 70%.