Abstract: We propose a new method for reconstructing complex surface velocity field in weir flow which is notoriously difficult to measure due to its sharp drop and high fluctuation. The new method is able to map the curved flow surface by two coplanar cameras and achieves measurement of two-dimensional velocity field through large-scale particle image velocimetry (LSPIV). The measured two-dimensional velocity field is projected onto the three-dimensional flow surface based on complex linear mapping relationship between the object space and image space. Application of the method to weir flow measurement yields satisfactory results, showing that the longitudinal velocity turns in trend from increase to decrease at the middle point of nappe, while the vertical velocity increases all the way along the nappe and plays a dominant role in the lower half of the nappe.