Abstract: A regulating valve is the core equipment of pipeline transportation in the petrochemical industry. It can control the technological parameters of pipeline medium (e.g. flow rate, pressure, temperature and liquid level) by changing the valve position of its own. In this paper, one pressure reducing station of a certain products pipeline in China was taken as the example. After the pressure reducing valve at the station was in operation for five years, its regulating instability occurred in the PID control mode. And the direct cause of this fault is that the lower part of the labyrinth valve cage is damaged. Then, this unstable operation behavior was analyzed thoroughly by exploring the PID control principle of pressure reducing valve, combined with the operation characteristics of the valve and the performance parameters of the valve actuator. Besides, the specific solutions were proposed. It is indicated that the control stability of pressure reducing valve after the maintenance can meet the needs of pipeline operation completely. The research results can be used as the reference for the maintenance of similar faults and even the localization of pressure reducing valves.