This paper takes the propylene refrigeration compressor system of an ethylene plant as an example, and analyzes the problem that the liquid level in the first stage suction tank continues to be high during actual operation. After field investigation and discussion, it is concluded that the root cause of the difference between design and actual operation is the internal leakage of the temperature control valve on top of the first stage suction tank. During the operation of the ethylene plant, such measures as making process adjustment and installing a new pipeline for transporting hot propylene gas were taken to effectively solve this problem, thus ensuring the safe, stable and long-term operation of the propylene refrigeration compressor.