The invention discloses a system for treating malodorous gas. The system comprises a washing unit, a low-temperature plasma pre-oxidation unit and a catalytic-oxidation washing unit which are sequentially connected in series, wherein the washing unit comprises a spray washing tower; the low-temperature plasma pre-oxidation unit comprises at least one low-temperature plasma purifying bin; and the catalytic-oxidation washing unit comprises a catalytic oxidation tower, a filling layer is arranged at the upper part of the catalytic oxidation tower, a catalyst bed layer is arranged at the lower part of the catalytic oxidation tower, a gas inlet is formed in the middle of the catalytic oxidation tower, and spraying equipment is arranged at the top of the catalytic oxidation tower. According to the system, low-temperature plasma equipment and catalytic-oxidation washing equipment are combined in use, and the low-temperature plasma equipment plays a role of pre-oxidation, so that the water solubility of an organic matter is increased, meanwhile, the load of a subsequent catalytic-oxidation washing tower is decreased, and the operation cost of an oxidant can be greatly lowered; and secondary wastewater pollution in a washing process is avoided, the whole waste gas treatment process is simple, efficient and stable, the investment cost and the operation cost are low, and the system has very good practicability.