In order to improve measurement precision and sensitivity of metal linear expansion coefficient，thus simplify digital measurement and computer processing，the micrometric displacement is completed by using the core element of PSD photoelectric sensor. An optical enlargement device is employed to advance micromovement resolution ratio. The chip BOS-PSD0018 is employed to achieve micromovement conversion. A microprocessor is utilized to realize digital procession and display for metal linear expansion coefficient. The standardization test is completed by using a copper rod; the detection test is conducted by using iron rod. The results show that the maximum relative error of metal linear expansion coefficient is less than ±2. 3%，the optical enlargement device can 　　increase 4 times for the resolution ratio of micromovement.