Co-oscillating tides in the Yellow Sea south of Shantung Peninsula are investigated analytically by a superposition of Kelvin and Poincaré waves. The Yellow Sea is approximated by a rectangular bay of uniform depth with an opening at the head, and a variable portion of tidal energy is allowed to penetrate through the opening. The analytical results basically agree with the available tidal charts. For the semi-diurnal tide the Poincaré waves play an important role throughout the whole basin of the Yellow Sea, but for the diurnal tide their influence is restricted to the vicinity of the bay head. The asymmetry of amphidromic system arises primarily due to a partial penetration of tidal energy through the opening at the bay head. A large tidal elevation in the Kyunggi Bay south of Ongjin Peninsula is due to the modifications of Kelvin and Poincaré waves at the Ongjin Peninsula.