During normal operation, the shaft vibration at the high and low pressure ends of cracking gas compressor turbine in the ethylene plant at PetroChina Liaoyang Petrochemical Company fluctuated abnormally. By checking the process equipment and other factors, it was determined that the abnormal vibration was caused by oil seal coking. During unit maintenance, the oil seal structure at the high and low pressure ends was analyzed, and isolation nitrogen was introduced into the oil seal, thus fundamentally solving the problem of oil seal coking.