On-chip erbium-doped/erbium-ytterbium co-doped waveguide amplifiers (EDWAs/EYCDWAs) have received extensive research attention in recent years, As an important component of EDWA, there has been relatively little research on integrated wavelength division multiplexing/demultiplexing devices for 980 nm pump light and 1550 nm signal light. This article aims to propose a compact Ta₂O₅ 980/1550 nm wavelength diplexer based on multimode interference effects. The device employs a structure of symmetric interference and paired interference cascade, which reduces the total length of the segmented multimode interference waveguide to one-third of the ordinary paired multimode interference waveguide without using any complex structures such as subwavelength gratings to regulate the beat length of the pump and signal light. The three-dimensional finite difference time domain (3D-FDTD) tool was 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 the extinction ratios are both better than 16 dB. Moreover, the 1-dB bandwidth is up to 150 nm around the 1550 nm wavelength and up to 70 nm around the 980 nm wavelength. The segmented structure designed in the article greatly reduces the design difficulty of MMI devices and reduces the overall size of 980/1550 nm wavelength division multiplexers/demultiplexers. It is expected to be applied in on-chip integrated erbium-doped waveguide amplifiers and lasers. In addition, the segmented design approach of cascading the hybrid multimode interference mechanism provides a technical reference for separating two optical signals with far apart center wavelengths such as 800/1310 nm and 1550/2000 nm, and has potential application value in communication and mid infrared diplexing devices.