Using long-term satellite altimeter data, a new streamline-based algorithm is developed to identify the Kuroshio intrusion types and describe the seasonal variations of related dynamical properties. Results from this new classification show that a mixing of leaping, looping and leaking streamlines is the dominant form of Kuroshio intrusion into the South China Sea (SCS). The leaping path is very stable and crosses the Luzon Strait mainly through the Balintang Channel regardless of seasons, while the streamlines leaking into the SCS is more likely to intrude via the channel between the Babuyan Island and the Camiguin Island. Large seasonal variations are found with the percentage of each kind of streamline and the Luzon Strait Transport (LST), but not with the intensity, width and current axis position of the Kuroshio. The along-streamline analysis reveals that the seasonal intrusion of the Kuroshio is essentially the seasonal variation of the cyclonic shear part of the flow. A possible physical mechanism is proposed to accommodate these seasonal characteristics based on globally the vorticity (torque work) balance between the basin-wide wind stress and the lateral friction, as well as locally the loss of balance between the torques of interior stresses and normal stresses both provided by the wall boundary, together with a plausible conjecture that the seasonally-reversing monsoon can significantly modify the torque of the interior stresses within the cyclonic shear part of the flow and thus responsible for the seasonal variation of the Kuroshio intrusion.