An 800 kt/a ethylene plant adopts domestic cracking technology and environmentally-friendly low-energy-consumption ethylene separation technology. The separation process is introduced, and design optimization is carried out such as installing closed steaming in quench system, reducing the pressure in ethylene rectification tower, directly sending ethylene to downstream units, reducing the emission of ethylene spherical tank and recovering cold energy from low-temperature raw materials. These optimizations can effectively improve the flexibility and progressiveness of ethylene separation process, reduce the unnecessary emissions, reduce the operation cost, and achieve a more environmentally-friendly and energy-saving production of the plant.