The Y-branch and multimode interferometric(MMI) structure are used in many kinds of planar lightwave circuits, and the symmetrical output of Y-branch or MMI structure takes important role in these apparates. The main factor that introduces asymmetry into the output of Y-branch or MMI structure is the first-order mode guided by them. A coupler in which the waveguides which can only guide the fundamental modes are located on both sides of the multimode waveguide is designed, and this coupler can filter out the first order mode guided by it. This coupler is used in Y-branch and MMI structure, and the simulation results show that the output of Y-branch and MMI structure with this coupler has better symmetry in comparison with traditional structure.