The invention relates to a method and device for accurately detecting the height of the used lead-acid battery bus bar. The method comprises the steps: the used lead-acid batteries are destacked in the early stage, after the lead-acid batteries are put on the work line and in the device of the invention, the lead-acid batteries are stopped on a fixed position to be scanned by an X-ray generator, the height information of a lead-acid battery bus bar is obtained through the image processing, and then the information is sent to a cutting machine so as to accurately separate the top covers and the bodies of the used lead-acid batteries in the later stage. The device of the invention comprises an X-ray generator which can accurately find the cutting height, so that the each separated substance of the used lead-acid batteries has higher purity than before; the X-ray generator and a linear array detector move together, so that two lead-acid batteries can be scanned through one stroke including moving down to a lower limit position and moving up to return, the work efficiency is greatly improved, and the energy is saved.