A Y-junction is commonly used in on-chip systems because of its excellent broadband characteristic. However, due to the lack of regulation methods, in most cases Y-junctions are used as passive components. In this work we propose a reconfigurable broadband Y-junction based on phase change material. When Sb2Se3 layers on two branches are at different states, the Y-junction is asymmetric and works as a reconfigurable dual-mode (de)multiplexer. When both Sb2Se3 layers are amorphous, the Y-junction is symmetric and works as a dual-mode 3-dB power splitter. To achieve quasi-adiabatic evolution for both states in a short device length, we propose a segmented fast quasi-adiabatic method. By dividing the gap region into multiple segments and optimizing the geometry and length of each segment, the proposed device achieves bandwidth > 100 nm (crosstalk < -20 dB) in a compact footprint of 19.3 × 3 µm2. The simulation result shows that at center wavelength of 1550 nm, the crosstalk and insertion loss of our device are < -41 dB and <0.12 dB, respectively, under asymmetric mode (de)multiplex state, and the excess loss is within 0.06 dB under symmetric power splitting state. The proposed device may contribute to the realization of a high-bandwidth, flexible mode-division-multiplexing network.