This study proposes a new technique for surface measurement by using fringe-based optical flow. The principle of measuring surface shape by optical flow method is introduced, and the changes of projected fringes are analyzed from the perspective of optical flow. The theoretical relationship among the optical flow, height distribution, and phase distribution of the measured surface is established under parallel projection. A numerical simulation conducted with an established spherical crown geometric model shows that the optical flow method can be used to directly calculate the height of the measured object. Practical measurement of the object and comparison of the measured results with those of the phase shift method reveal that the optical flow method can accurately restore the object's phase; in addition, it demonstrates good robustness to the noise emanating from the void area of the measured object. Unlike traditional surface shape measurement techniques, the optical flow method only needs two frames to accurately restore height or phase distributions. Because the optical flow method itself contains the time factor and only requires two images to directly obtain surface shape distribution, it is more suitable for dynamic measurement than the phase shift method.