River surface imaging velocimetry is a nonintrusive flow measurement technology, in which the flow field calibration mainly uses the Direct Linear Transform (DLT) method. Since DLT is greatly limited by the quantity, distribution and accuracy of ground control points (GCPs) set on the river banks, the current measurement systems cannot layout fast and safely under complex field conditions. To handle this problem, a flow field calibration method based on ObjectImage Scaling (OIS) is proposed. An imaging model of variableheight plane under oblique angle is built by introducing the tilt of camera and variation of water level. Then, a “pointsdistance” transformation between image plane and river surface is derived. Finally, a measuring device assisted by a laser range finder is designed and calibrated to obtain the model parameters. Experimental results show that the accuracy of OIS is close to the DLT, and no control points are needed to be deployed or surveyed with a total station, which significantly improves the efficiency and security of the field work. This study is expected to provide theoretical and technical support for the design of new integrated optical instrument for river flow measurement.