The cracking gas compressor in the No.2 ethylene plant at PetroChina Daqing Petrochemical Company was not designed for nitrogen operation. Aspen Plus was used to build a nitrogen operation model for cracking gas compressor, and simulation was carried out by using the performance curve of the compressor to calculate the cooling capacity of CW cooler and the source of nitrogen during nitrogen operation. The nitrogen operation of cracking gas compressor was achieved for the first time, and the high pressure system was air sealed, so as to achieve the goal of zero leakage during the startup with real material.