The operation difficulty of debutanizer is one of the bottlenecks in the later stage of operation for many ethylene plants. In order to solve this problem, the process technology is transformed to direct the discharge of low-pressure depropanizer to tank farm, the debutanizer is isolated for treatment, and then the off-spec mixed C₄ is recycled to the debutanizer for refining. In this way, the debutanizer can be isolated for treatment without shutting down the ethylene plant, thus ensuring the long-term operation of ethylene plant.