To fulfill the urgent demand for the miniaturization and cost reduction of fiber-optic gyroscopes in inertial navigation systems, an integrated optical engine chip for fiber-optic gyroscopes based on high-aspect-ratio silicon-nitride waveguides is proposed and designed. This chip offers on-chip functions such as light-source coupling, beam splitting, transmission, and polarization. The optical engine chip mainly comprises two 1×2 multimode interference (MMI) beam splitters and polarizers. The important parameters of the chip were simulated and calculated using Luminary FDTD to determine the size of the MMI beam splitter, which is 15 μm × 200 μm. The polarization extinction ratio can reach 33 dB and the splitting is uniform. Finally, a low-loss processing method for silicon-nitride optical engine chips is devised. The designed high-aspect-ratio silicon-nitride optical engine chip serves as a solid foundation for the miniaturization and integration of high-precision fiber-optic gyroscopes.