The invention relates to application of a 4-methoxy-indole solution in control of tobacco black shank, and a preparation method of the 4-methoxy-indole solution. The 4-methoxy-indole solution is used for controlling the tobacco black shank. The preparation method of the 4-methoxy-indole solution comprises the steps of feeding an OP-10 type emulsifying agent into 4-methoxy-indole, wherein the volume of the emulsifying agent accounts for 10-40% of the total volume; dissolving by dimethylbenzene so as to prepare the 4-methoxy-indole solution with the content of 10-40% (w/ w). The application of the 4-methoxy-indole solution in control of the tobacco black shank has the beneficial effects that 1. 4-methoxy-indole has the characteristics of being capable of inhibiting the pathogenic bacteria of the tobacco black shank and is high in biological activity, thus having a good control effect on the tobacco black shank; 2. a brand new understanding of the use of the 4-methoxy-indole is provided, and a new bactericide is provided for controlling the tobacco black shank.