An essential part of coherent optical communication and heterodyne detection, the 90°optical mixer improves polarization discrimination and anti-interference capabilities, increases receiver sensitivity, and permits demodulation of higher-order modulation forms. The disadvantages of conventional 90° optical mixers, however, include their high precision needs, size, mode mismatch restrictions, polarization sensitivity, and single functionality. Utilizing a lithium niobate platform, a Multimode Interference (MMI) structure, and a micro-thermoelectric electrode array, this study uses the Finite Difference Time Domain (FDTD) method to design a multipurpose device that combines 90°optical mixing and mode separation capabilities. The multipurpose device acts as a 90° optical mixer when no voltage is provided across the micro-thermoelectric electrode, according to the results. All four output ports have common-mode rejection ratios above -30 dB, phase errors below 4°, and losses surpassing -13.862 dB in the 1520–1580 nm wavelength region. TE0,TE1,TE2,and TE3 modes are separated by the multipurpose device acting as a mode splitter when a voltage is supplied across the micro-thermoelectric electrodes. In addition to controlling crosstalk fluctuation within 8.8 dB, the minimum loss divergence between modes is less than 0.024 dB. The physical characteristics of optical field interference within the MMI structure allow for perfect phase matching and energy distribution throughout a wide spectrum range, according to research findings, even when no voltage is supplied across the micro-thermoelectric electrode terminals. By controlling the interference superposition process inside the multi-mode region and improving broadband 90° optical mixing parameters, stable phase-matching conditions are maintained across the wide spectrum.The lithium niobate-based linear electro-optic effect (Pockels effect) modifies the waveguide refractive index distribution through an external electric field when a voltage is placed across the micro-thermoelectrode. By changing the light field's coupling path and propagation mode inside the MMI structure, this allows the mode-separating integrator to precisely achieve mode separation. The efficiency of the electro-optic effect in optical functional control is confirmed by this, which meets the isolation requirements for various mode optical signals. Furthermore, a methodical tolerance analysis of the device's width and length was carried out, demonstrating how structural dimensional deviations affect the mode coupling efficiency and optical field interference circumstances. The integrated broadband 90° optical mixer and mode splitter device described in this paper has excellent process tolerance properties.