The invention belongs to the field of agricultural agents, and provides an agent composition capable of being used for preventing and curing black rot of tobacco roots and application thereof. The agent composition is prepared from an active ingredient A and an active ingredient B, wherein the active ingredient A is sodium selenite, and the active ingredient B is prochloraz or carbendazim; the weight ratio of the active ingredient A to the active ingredient B can be 1:(1-50). The agent composition has a very good effect of preventing and curing the black rot of the tobacco roots and can effectively retard the agent resistance and reduce the agent applied dosage.