The loss properties of multimode Υ-branch waveguides are analyzed using coupled mode theory combined with two-dimensional Padé (1,1) approximation finite-difference beam propagation method. The radiation losses of zeroth-order mode propagating in single-mode and multimode turning channel waveguides are compared. Mode conversion in the symmetric five-layer waveguide for multimode Υ-branch configuration is analyzed. Then the relations between radiation loss and the branching angle for single-mode and multimode Υ-branch waveguides are obtained and explained. It is found that when there is not much coupling from zeroth-order mode to other higher-order modes in a multimode Υ-branch waveguide, radiation loss of zeroth-order mode propagating in the waveguide is lower than that in a single mode one. For a multimode Υ-branch waveguide, the radiation loss curve fluctuates as the result of mode coupling and influence of the turning points of the waveguide structure. By making use of the fluctuation of the radiation loss curve, optimizing the structural parameters such as branching angle and the width of waveguide and trying to eliminate the generation of the second-order mode at the turning points, a design of a multimode Υ-branch waveguide with low loss and wide branching angle is obtained.