The invention relates to an outdoor acoustic-electricity joint positioning method for a multi-rotor unmanned aerial vehicle for transmission line inspection. An acoustic-electricity joint transmittingunit is installed on a transmission line tower, and an acoustic-electricity joint receiving unit is installed on the unmanned aerial vehicle. After the acoustic-electricity joint transmitting unit emits electromagnetic wave signals and ultrasonic signals, a circular ultrasonic sensor array in the acoustic-electricity joint receiving unit receives the ultrasonic signals, and an electromagnetic wave sensor receives the electromagnetic wave signals. A signal processing and positioning module calculates the distance between a transmitting source and a receiving array according to the time difference between the electrical signals and the ultrasonic signals and the transmission speed of the ultrasonic waves in the air, performs spatial spectrum estimation operation on the signals received by the ultrasonic array to obtain the incoming wave direction of the ultrasonic transmitting source, and determines the position of the transmitting source in the coordinate system of the unmanned aerialvehicle based on the direction and distance of the incoming waves. The invention realizes fast and accurate positioning of unmanned aerial vehicles at a low cost, and can provide a positioning basis for inspection of transmission lines of unmanned aerial vehicles.