Abstract: In shallow ocean, sound transmission is mainly dominated by seafloor reflectivity. Utilizing the prerequisite that the slope of bottom loss versus grazing angle can be considered as a single parameter F_dB to describe sea bottom, the reflection phase angle can be determined via a simple approximation based on the incidence of plane wave to a uniform fluid half-space. The interference field, in which F_dB and reflection phase angle can be inputted to computation models as boundary conditions, is estimated. And the sphere of application is extended to the case of complex sound speed and range-dependent environment. The validity of the single parameter model that includes reflection phase is illustrated by the simulations for three types of seabed examples, in which the transmission losses predicted by basic parameters are compared with those predicted by this simple seabed model.