Automatic synchronization is one of the important functions of marine power station management system. Grasping of ACB (air circuit breaker of generator) closing opportunity is the difficult and key point of automatic synchronization control. This paper offers a rapid and simple method of automatic synchronization control based on PLC and emphasizes on discussing a new method of grasping ACB closing opportunity in synchronization control. The system researched makes the best use of the computing function of PLC program and reduces the hardware as possible to improve the reliability and economical efficiency. Similar triangle principle and a new timing mode based on PLC scan cycle are adopted in grasping ACB closing opportunity. Several years of operation in training lab proved that the automatic synchronization system is reliable and steady.