A technique based on L-K local optical flow algorithm for retrieving the height distribution of a measured surface is proposed by the calculation of the coordinates between the capture line and projection line in space. The measuring system is composed of a CCD camera and a projector which project a grating pattern with a small angle. The height distribution of a measured surface can be calculated directly by using the intersection coordinates of the projected line and the captured line, where the changes of the observation position between two fringe patterns caused by the measured surface can be calculated by L-K optical flow algorithm. The theoretical relationship between the optical flow and the height of the measured object at the intersection of the projected light and the line of sight under the projection condition of the point source is established. Simulation and experimental results show that the proposed method can retrieve the height of the measured object surface accurately. Different from traditional shape measurement methods, the proposed surface shape measurement technique using captured line and optical flow does not need to collect multiple fringe images, nor need to calculate the fringe frequency and the phase distribution, but only needs two fringe images to retrieve the height distribution of the measured surface.