Abstract: In order to extend the velocity application range of the two-layer Boussinesq model with the highest spatial derivative of 2, a new function with five parameters is constructed in this study for the horizontal and vertical velocities of the water body, to describe the velocity characteristics along the water depth direction. The nonlinear least square method is used to fit the velocity field of this model, so as to optimize the values of these parameters. To test the effectiveness of this method, the velocity fields of Stokes linear wave, of the stream-function, and of the focused wave are selected for comparison. The research results show that this method can effectively reduce the error in the velocity field of the equation and broaden the applicable velocity range by approximately three times. By comparing the stream function across the entire wavelength with the velocity field formed through superposition of multiple kh linear waves, the applicability of this approach has been further validated.