The invention relates to an air cylinder head water-channel heat radiating structure of a water cooling engine, which is applied to an air cylinder head of a water cooling engine. A plurality of air valve seats and a cooling water channel are arranged in the air cylinder head, each air valve seat is respectively used for holding an air inlet valve or an exhaust valve, the air valve seats include at least two air valve seats which are closely adjacent to each other, the distance between the two adjacent air valve seats is smaller than that among the surplus air valve seats, and the cooling water channel can allow the cooling liquid to flow by the outer edge of the air valve seats and to exchange heat with the air valve seats. The heat radiating structure comprises a thin wall part and at least one heat radiating rib element; the thin wall part is a thick outer edge which is arranged between the two adjacent air valve seats, so that the cooling liquid cannot flow by between the two adjacent air valve seats; the heat radiating rib element is formed by extending the thin wall part towards the cooling water channel, so that the heat radiating rib element can stretch into the cooling water channel, and the cooling water channel is kept smooth. Therefore, a purpose of rapidly dissipating the heat, improving the integral structural strength and improving the engine performance can be achieved.