The utility model provides a horizontal bioreactor for exhaust gas treatment, which comprises a groove, a first carrier and a first washing device. The groove is provided with a gas inlet and a gas outlet and comprises a first washing area and a first bio-decomposition area. The first washing area is divided into a plurality of first washing chambers that are communicated with each other. The first bio-decomposition area is arranged under the first washing area and communicated with the first washing area. The first carrier is arranged in the first washing area and the first bio-decomposition area. The first washing device is arranged at the top of the groove for providing a first bacterial liquid which flows to the first bio-decomposition area to the first washing area. The exhaust gas enters to the groove from the gas inlet and drains out of the groove from the gas outlet in the direction substantially parallel to the earth.
