The invention belongs to the technical field of plant protection, and relates to a tobacco-black-shank-resisting and tobacco-bacterial-wilt-resisting bio-control strain Trb3. The bio-control strain is a bacillus subtilis. A method for preparing a microbial agent by using the strain Trb3 comprises the steps that: the strain Trb3 is subjected to oscillation culturing in an LB culture solution for 36-40h under a speed of 180rpm under temperature of 28 DEG C; the obtained material is centrifuged for 10min under a speed of 6000rpm; the material is diluted by using sterile water, such that the microbial agent is prepared, wherein a viable bacteria total concentration in the microbial agent finished product is 1*10<9>-1*10<10>u/ml. The Trb3 strain microbial agent is applied for root dipping before tobacco transplanting or for root irrigating during tobacco transplanting, and for root irrigating after tobacco transplanting and right before disease periods of tobacco black shank and tobacco bacterial wilt. In a greenhouse, a controlling effect of the Trb3 strain microbial agent against tobacco black shank reaches 72.3%, and a controlling effect of the Trb3 strain microbial agent against tobacco bacterial wilt reaches 65.7%. When the Trb3 strain microbial agent is applied in field, a controlling effect of the Trb3 strain microbial agent against black shank and tobacco bacterial wilt is 61.04%.