The invention discloses an aqueous urea solution decomposing mixing device. A spoiler and a pore plate are arranged between a front shell and a rear shell. The spoiler and the pore plate divide an inner cavity into an upper spraying cavity, a middle flow guide cavity and a lower rotational flow cavity. A top cover is arranged at the top of the rear shell. An inclined plane is arranged in the spraying cavity, and an included angle is formed between the vertical plane and the inclined plane. A flow guide plate is provided with a flow guide vane. The spoiler is provided with a turbulent flow vane. A rotational flow plate is provided with a rotational flow vane outwards through a third vane hole. The inner cavity of the aqueous urea solution decomposing mixing device is divided into the spraying cavity, the flow guide cavity and the rotational flow cavity only through the spoiler and the pore plate. Little blocking occurs in an airflow channel, the airflow channel is smoother, mixed airflow passes through few turning points in the airflow channel, energy loss is small, and the airflow pressure drop is low. The top cover is provided with an inclined lower plane, and the spraying cavityforms a cavity which is asymmetric along the vertical plane. A nozzle seat is located in a low position of the lower plane, and thus the urea spraying line is toward the direction directly facing thedirection of the turbulent flow vane of the spoiler.