Advanced powertrain technologies have improved engine performance with higher power output, lower exhaust emission, and better controllability. Chief among them is the development of spark-ignition direct-injection (SIDI) engines in which the in-cylinder processes control the air flow motion, fuel–air mixture formation, combustion, and soot formation. Specifically, intake air with strong swirl motion is usually introduced to form a directional in-cylinder flowfield. This approach improves the mixing process of air and fuel as well as the propagation of flame. In this study, the effect of intake air swirl on in-cylinder flow characteristics was experimentally investigated. High-speed particle image velocimetry (PIV) was conducted in an optical SIDI engine to record the flowfield on a swirl plane. The intake air swirl motion was achieved by adjusting the opening of a swirl ratio (SR) control valve which was installed in one of the two intake ports in the optical engine. Ten opening angles of the SR control valve were adjusted to produce an intake SR from 0.55 to 5.68. The flow structures at the same crank angle degree (CAD), but under different SR, were compared and analyzed using proper orthogonal decomposition (POD). The flow dominant structures and variation structures were interpreted by different POD modes. The first POD mode captured the most dominant flowfield structure characteristics; the corresponding mode coefficients showed good linearity with the measured SR at the compression stroke when the flow was swirling and steady. During the intake stroke, strong intake air motion took place, and the structures and coefficients of the first modes varied along different SR. These modes captured the flow properties affected by the intake swirl motion. Meanwhile, the second and higher modes captured the variation feature of the flow at various CADs. In summary, this paper demonstrated a promising approach of using POD to interpret the effectiveness of swirl control valve on in-cylinder swirl flow characteristics, providing better understanding for engine intake system design and optimization.