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Stepless capacity control technology for reciprocating compressors is a key contributor to energy saving for the petroleum and petrochemical industries. Devices called 'unloaders' are utilized to control the capacity of the compressor by forcibly holding the suction valves open during a variable portion of the compression stroke to control the compressor output. This approach can also lead to various faults of the suction valve. This paper describes the simulation and experimental studies of the transient motion of suction valves under stepless capacity control. Beginning with mathematical models for the normal cycle, improved models of a reciprocating compressor under stepless capacity control have been built. A simulation study of the working process of a double-acting reciprocating compressor has been completed. Theoretical formulas for the transient motion of the valve plate under complicated conditions and the dynamic pressure in the cylinder are compared with the experimental results. Based on the above simulations, a finite element analysis of the valve plate and valve seat has been completed. The experiment results showed that the vibration of the compressor cylinder under complicated conditions was consistent with numerical simulation results. Research presented in this paper is significant in providing tools for diagnosing faults in order to optimize the design of reciprocating compressors that utilize a stepless capacity control system. [ABSTRACT FROM AUTHOR]