Abstract Preferential flow within soil cracks influences land surface hydrological processes, yet direct quantifying preferential flow velocity (PFV) remains challenging. Here, we develop a method for high‐resolution monitoring and quantifying PFV in soil cracks using optical frequency domain reflectometry (OFDR). Building on the flow characteristics of preferential flow, we further establish a quantitative framework that links crack width to PFV through a physically based drag‐partitioning mechanism. Laboratory experiments and field slope monitoring enable continuous and accurate measurement of PFV and reveal a coupled evolution between crack width and PFV during flow flushing. This coupled behavior is interpreted from a dynamical perspective based on redistribution of viscous and form drag. By mechanistically relating PFV to readily measurable crack width, the proposed framework provides a parameterization pathway for improving hydrological models. Incorporating crack‐scale PFV enhances the representation of infiltration and other surface hydrological processes.