A scheme of mould opening degree auto-detecting device is proposed. The inner-wall dimension of mould working segment can be measured and the opening degree can be calculated by using the auto-detecting device. Due to the mechanical structure design the off-line measurement in mould opening degree is realized. The paper analyzed the factors which causing error and calculated the error corresponding to these factors, so the total error is obtained. The hardware and software system is designed based on VC++ GDI. The measurement result is displayed in screen using curve mode.