The invention relates to unmanned aerial vehicles and provides a four-way-extensible paddle protecting cover which comprises two platy shells. A cavity capable of containing a paddle rotating device is formed in each shell, a position, corresponding to the paddle rotating device, of each shell is provided with a window, one shell can be rotated onto the other shell through a connecting member to be superposed, each shell comprises a hollow fixed portion and at least one sliding portion, each sliding portion and the corresponding fixed portion surround to form the corresponding cavity, and each sliding portion partly slides to extend into the corresponding fixed portion in a direction parallel to the rotating axis of the corresponding shell. The invention further provides a power system which comprises the protecting cover. The invention further provides an unmanned aerial device which comprises the power system. Each paddle portion extends along the rotating axis perpendicular to the corresponding shell when stopping by controlling the corresponding paddle rotating device, and the protecting cover is enabled to contract in a direction parallel to the rotating axis while superposing around the rotating axis, so that structural size of the unmanned aerial vehicle is reduced greatly and the unmanned aerial vehicle is convenient for a user to carry.