Space-Time Image Velocimetry (STIV) is a time-averaged velocity measurement method. It takes a testing line as the Interrogation Area (IA), and detects the texture orientation of a generated Space-Time Image to estimate the 1D velocity. STIV has great potential in real-time monitoring of river flow owe to its high spatial resolution and low time complexity. However, its practicality is still limited by the deficiencies of sensitivity analysis and uncertainty evaluation due to the complexity of field conditions and the variability of river flow. To handle this problem, a rolling painting device is designed to simulate the movement of river surface flow, and the quantitative evaluation of a DFT-STIV method is comprehensively conducted. Compared to the evaluations based on numerical simulation and field measuring, it provides relatively real measuring environment and stable motion parameters. Experiment results show that the length of testing line and the tracing condition are main sensitive factors of velocity measurement. With reasonable parameter settings, the uncertainty of STIV can be controlled within ±1%, which meets the accuracy requirement of hydrometry.