In order to reduce the branching loss,a Y-branch waveguide structure is presented,in which a multimode tapered transitional waveguide is embedded between an incident channel and an output branch. Compared with a rectangular transitional waveguide which has been introduced in Y-branch for the same purpose,the tapered transitional waveguide can not only reduce the length of the transitional waveguide,but also further reduce the branching loss. The annealed proton-exchanged LiNbO3 Y-branch optical waveguide with a tapered transitional section is simulated by the three-dimensional finite difference beam propagation method. The numerical results are useful for the design and fabrication of this kind of Y-Branch waveguide.