Abstract: The bi-directional flow often happens in the downdraft reach due to the influence of main thannel,so that its velocity calculation is obviously different from normal river in practical engineering.In many condition,the necessary hydrologic data and topography is lacked in these downdraft reaches,then the mathematical and physical model are difficult to apply directly.So based on the combination of physical, mathematical model and multivariate regression analysis for typical downdraft reach,a empirical formula is putted forward in this paper,and can be directly used to determine maximum velocity at a engineering point in downdraft reach.