Isolated phase bus (IPB) is chosen as the structure type of the enclosed ac busbar for ITER poloidal field (PF) converter, which is used to connect the rectifier transformer and the converter unit of the ITER PF converter module due to its electromagnetic shielding ability. To solve the heating problem of the high-power IPB, water cooling method is adopted. This paper presents the thermal design of high-power water-cooled ac enclosed busbar which is applied for ITER PF converter. The proposed approach takes full advantage of the quickness of the analytical method based on the heat transfer theory and the accuracy of the 3-D finite element method, focusing on the predictions of the temperature rise of circulating water and the enclosed busbar. Temperature measurements from the test based on a full scale prototype of the IPB show good agreements with the predictions, which proves the high efficiency and accuracy of the proposed approach in the design of cooling scheme for the ten-kilowatt-class enclosed ac busbar.