Abstract: In order to solve the problems of magnetic flux leakage, heating and cooling in the end region of large synchronous condenser for ultra high voltage (UHV) transmission, a three-dimensional transient electromagnetic field mathematical model was established based on the actual operation characteristics of the synchronous condenser, and the distribution law of magnetic flux leakage in the end and eddy current loss of each component was studied. By constructing a three-dimensional fluid-solid coupling heat transfer model in the end of synchronous condenser, the heat transfer law of the cooling medium and the heat transfer law of the outer shielding space and the inner shielding narrow space of the lower ventilation system with limited space constraints were studied. Furthermore, the effects of different cooling media on the thermal characterization of the new dual stage copper shield and convex clamping plate were investigated. In order to verify the rationality of the numerical analysis model and the effectiveness of the calculation method, the temperature of the inner edge of the copper shield in the end of the synchronous condenser was measured. The temperature calculation results were consistent with the temperature measurement results. It provides a theoretical basis for the improvement of the cooling effectiveness of the synchronous condenser ventilation and cooling system, and the research and development of the synchronous condenser cooling system for UHV power transmission with larger capacity, and the selection of the optimal cooling medium.