The invention provides a four-rotor UAV (Unmanned Aerial Vehicle) obstacle avoidance device and method based on infrared light ranging. The UAV comprises an infrared light emitting and receiving module, an ultrasonic ranging module, a steering gear and plane mirror rotating device, and a main control chip. The obstacle avoidance method comprises the following steps of installing the infrared lightemitting and receiving devices above the end of a front frame arm and below the end of a rear frame arm of the UAV, wherein the four infrared light emitting and receiving device can form a rectangleby passing through the points reflected by the obstacle, that is, a plane pattern of the UAV; realizing the function of allowing the infrared light to scan out a certain angle along the diagonal direction of the rectangle by using the combination of the steering gear and the plane mirror; determining the distance result of scanning; and if the obtained distance information exceeds a certain threshold, determining that the UAV moves to the corresponding diagonal reverse extension line to avoid the corresponding obstacle. The infrared light emitting and receiving module is inexpensive and meetsmost of the work requirements compared with other modules.