The present invention relates to the field of air vehicle technologies and provides an arm, a power assembly and an unmanned aerial vehicle. The arm includes a principal arm and an auxiliary arm. The principal arm is mounted on the vehicle body and the principal arm can rotate relative to the vehicle body. One end of the auxiliary arm is connected to the principal arm. The auxiliary arm can rotate relative to the principal arm. In the foregoing manner, an unmanned aerial vehicle having the arm is compact in structure, small in volume and easy to carry after being folded.