For precision machinery, the measurement of the relative in-plane displacement of two parallel planes that are separated by several meters is important. In this paper, a theoretical model for measuring the relative in-plane microdisplacement between two parallel planes was developed on the basis of secondary diffraction. Based on this method, we employed a pinhole and a circular-ring as the diffraction screens. The influence of the structural parameters of diffraction screens on the secondary diffraction pattern was analyzed in detail, and the obtained parameters were then used in the experimental measurements. For experimental investigation, a laser beam at 532 nm was used to irradiate a pinhole; the diffracted light was then further diffracted using a circular-ring, and the final diffraction pattern was recorded using a CCD camera. The circular-ring was mounted on the plane to be measured, while the pinhole and the CCD camera remained stationary; the space between the pinhole and the circular-ring was set at 1200 mm. The displacement of the circular-ring can be calculated by comparing the central position of the two diffraction patterns before and after shifting the circular-ring. Over a measurement range of 0-90 mu m, the absolute error in the displacement measurement was less than 1.97 mu m.