Magnetic rings are widely used in electronic manufacturing. In the production process, various types of cracks exist, which severely reduce product performance. So it has important practical significance for the efficiency and accuracy detection of the magnetic rings defects. In this paper, a detecting instrument for magnetic rings defects is designed based on magnetostrictive effect, which uses an alternating magnetic field to encourage the ring producing vibration at one end of the ring, and at the other end a vibration transducer is used to catch the vibration signal. Considering that signals of vibration are difficult to obtain, a programmable filter based on the MAX262 chip is designed, using fourth-order Chebyshev band-pass filtering method to extract the weak signal ,which is used to determine the ring cracks. Numerous ring experiments show that this method can effectively detect the ring crack.