The high resolving power camera was often used to improve the precision of measuring system although its price is high, because the accuracy of image measure was limited by the pixel resolving ability of camera. A method of one dimension gray level quadrature sub-pixel to detect edge was presented. This method was applied to measure the displacement of measuring head on 3-coordinate measuring machine. The data of test was shown that this system can distinguish displacement 0.001mm. it is higher accuracy than pixel level. So this method is convenient and fast , fit to track and measure the moving object. It is obvious that this method can save the cost of hardware.