On-chip erbium-doped/erbium-ytterbium co-doped waveguide amplifiers (EDWAs/EYCDWAs) have received extensive research attention in recent years. However, there has been relatively little research on integrated wavelength division multiplexing/demultiplexing devices for 980-nm pump light and 1550-nm signal light. This work aims to propose a compact Ta₂O₅ diplexer for 980/1550-nm wavelengths based on multimode interference effects. The device utilizes a structure that combines symmetric interference with a cascaded paired interference design, thereby reducing the total length of the segmented multimode interference waveguide to one-third that of a conventional paired multimode interference waveguide. This is achieved without using any complex structure, such as subwavelength gratings, to adjust the beat length of the pump and signal light. The three-dimensional finite difference time domain (3D-FDTD) tool is used to analyze and optimize the established model. The results demonstrate that the designed MMI diplexer has low insertion loss and high process tolerance, with an insertion loss of 0.4 dB at 980 nm and 0.8 dB at 1550 nm, and that the extinction ratios are both better than 16 dB. Moreover, the 1 dB bandwidth reaches up to 150 nm near the 1550 nm wavelength and up to 70 nm near the 980 nm wavelength. The segmented structure designed in this work greatly reduces both the difficulty in designing the MMI devices and the overall size of 980/1550 nm wavelength division multiplexers/demultiplexers. It is expected to be applied to on-chip integrated erbium-doped waveguide amplifiers and lasers. In addition, the segmented design method of cascading the hybrid multimode interference mechanism provides a technical reference for separating two optical signals with long center wavelengths such as 800/1310 nm and 1550/2000 nm, and has potential application value in communication and mid infrared diplexing devices.