A horizontal bioreactor is used to treat a waste gas and includes a tank body, a first carrier and a first washing device. The tank body has a gas inlet and a gas outlet and includes a first wash region and a first biodegradation region. The first wash region is divided into first wash rooms, and the first wash rooms are communicated with each other. The first biodegradation region is located under the first wash region and communicates with the first wash region. The first carrier is disposed in the first wash region and the first biodegradation region. The first wash region is disposed on the top portion of the tank body and inside the tank body to provide a first bacteria solution in the first wash region, and the first bacteria solution flows into and stays in the first biodegradation region. Wherein, the waste gas flows in the tank through the gas inlet and flows out the tank through the gas outlet along a direction substantially parallel to the ground.
