A motor shield covered on a motor of a drainage device over a cooling vane is disclosed to include a shield body having an arched portion and a straight portion, a guide wall disposed in the shield body and having an arched segment and a straight segment connected to the straight portion of the shield body, an airflow zone defined in the shield body in an interference relationship relative to the cooling vane and surrounded by the arched segment and straight segment of the guide wall and the arched portion and straight portion of the shield body, horizontally extending air outlets and vertically extending air outlets respectively located on the arched portion and straight portion of the shield body, and air inlets located on the periphery of the shield body. During rotation of the cooling vane, a flow of air is induced to carry heat out of the shield body through the vertically and horizontally extending air outlets efficiently.