The invention relates to the technical field of environmental protection, and discloses a poisonous gas collecting device. The poisonous gas collecting device comprises a rack and a collection box mounted on the rack, wherein the collection box is internally connected with a heating plate in a sliding manner; a mixing tank communicated with the collection box is fixedly arranged on the collectionbox; a high pressure spraying nozzle is arranged at the joint of the mixing tank and the collection box, and a nozzle of the high pressure spraying nozzle faces the collection box; the outer side of the mixing tank is sleeved with an air drum; an inner cavity communicated with the collection box is formed between the air drum and the mixing tank; the inner cavity is provided with a first one-way valve; the mixing tank is internally connected with a mixing shaft in a rotating manner; a water outlet hole is formed in the side wall of the mixing shaft; a pressing plate is arranged at the bottom of the mixing shaft; a second one-way valve is arranged on the pressing plate; an air bag is arranged on the air drum; a mixing box is arranged above the mixing tank; an opening for incoming water is formed in the upper part of the mixing shaft; the mixing tank is fixedly provided with a screw blade; a first air inlet pipe connected to a waste gas source is arranged on one side of the mixing tank;a first pressure valve is arranged at the joint of the first air inlet pipe and the mixing tank; a controllable cylinder is arranged on the mixing tank. The poisonous gas collecting device is simple in structure and convenient in gas treatment.