The scavenging process lasts for a fairly short period in the two-stroke marine engines. It not only transports the burnt gas out of the cylinder but also provides the fresh air for the next cycle, thereby significantly affecting the engine performance. In order to promote fuel-air mixing, the scavenging process usually generates swirling flow in uniflow-type scavenging engines. The scavenging stability, however, directly determines the scavenging efficiency and even influences fuel-air mixing, combustion and emission of the engine. In the present study, a computational fluid dynamics (CFD) analysis of the scavenging process in the steady-state scavenging flow test is conducted. Obvious precession phenomenon is found in the high swirl model, and Proper Orthogonal Decomposition (POD) method is used to analyze its multi-scale characteristics.