The effect of the Taiwan Strait Current on the onshore intrusion of Kuroshio, both contributing to the formation of Tsushima Warm Current, is addressed theoretically by invoking a geostrophic adjustment model previously proposed. The idealized model assumes two unbounded basins, shallow and deep, separated by an infinitely long and thin barrier. On either side of the barrier, a western boundary current in the deep basin and a shelf current in the shallow basin flow along the barrier with the surface elevation of the former higher than that of the latter. When a part of the barrier is removed and a gap is created, the onshore part of the western boundary current intrudes onto the shallow basin through the gap while conserving its potential vorticity. Both the intruding current and the shelf current will later geostrophically adjust themselves to the disturbances created by the intrusion. Model results show that the transport of onshore intrusion increases with the sea level difference imposed initially between the deep and shallow basins across the barrier, indicating that the sea level rise associated with the strengthening of shelf current inhibits the shelf-ward intrusion. The intruding current is in jet mode when its transport is maximized, which otherwise is in coastal mode. The maximization of transport occurs when the sea level difference between the two basins is sufficiently large. Although this model greatly idealizes the problem, it explains well the observed fact that the transport of Tsushima Warm Current is fed mostly by the Taiwan Strait Current in summer when the latter becomes the strongest, and by the onshore intrusion of Kuroshio in winter when the Taiwan Strait Current nearly vanishes, suggesting that the seasonal variation of the onshore intrusion of Kuroshio is largely due to the seasonal variation in the strength of the Taiwan Strait Current.