A power module includes a power source comprising an end defining a socket, an external power line comprising a plug inserted into the socket, a fixing piece fixed at the end of the power source, and a clamping piece. The clamping piece includes a base mounted on the fixing piece, two parallel fixing portions extending from the base and defining a slot therebetween, a hook positioned on one of the fixing portions and extending in the slot, and a clamping band. The clamping band includes a fixed end fixed on the base and a slide end. The slide end includes spaced fixing teeth, a fixing groove is defined between each two of the fixing teeth, the clamping band winds around the external power line, the hook is engaged with one fixing groove to lock the slide end, and the clamping band and the base cooperatively hold the external power line.