To solve the issue of  compressor surge of a large-scale ethylene plant using a cracking gas compressor unit during the nitrogen operating conditions, through several targeted test verifications and analysis,the operating characteristics of process gas compressor with nigtrogen gas were explored. At the same time, the reasons for the surge during the driving process of the large centrifugal compressor unit were analyzed and summarized.The study shows that compressor surge can be well solved by decreasing the resistance along the outlet pipe of comressor and providing extra antisurge valve bupass. And the results provide idea for running the large centrifugal compressor unit with nitrogen gas was proposed.