The transverse velocity formula of Engelund has been widely used internationally because it is strong theoretical.But improper assumptions were adopted when dealing with the bottom boundary conditions,which leads to the bad application to rough bed surface for the formula.This paper amended the slip velocity and then proposed the new form of transverse turbulent shear stress on the bed surface,and joined them together into the calculation of bottom boundary condition.By all of the efforts,the new transverse velocity formula which can be used in both rough and smooth bed surface was produced.The improved formula has universal applicability,and the calculated value is more consistent with the measured data than the previous formula.