With the development of large-scale hydro-generators, large hydro-generator cooling technology is increasingly demanded. Different cooling method will not only affect the structure of hydro-generators, also it will affect the energy consumption and reliability of the generators. The commonly large-scale hydro-generator cooling method includes: air cooling, water cooling, and evaporative cooling methods. This paper analyzes the principle of the three cooling methods and describes qualitatively the advantages and disadvantages of them. The air cooling hydro-generator structure is simple, but the generator operating temperature is high; the water-cooling method has a certain superior in cooling performance, but it requires more auxiliary equipments, and has higher equipment failure rates. The evaporative cooling method is a recently developed cooling technique. It not only has a remarkable cooling effect, but also can decrease the equipment failure rates and the cost of maintenance. In order to build a comprehensive model to assess the three hydro-generator cooling methods, this paper proposes a comprehensive evaluation method based on AHP. The method includes 11 indexes of resource consumption, energy consumption and reliability to assess the influence of cooling ways. The energy saving influence of all the 11 indicators are calculated by using the AHP comprehensive evaluation. Finally, comparison between a 400 MW air cooling and an evaporative cooling hydro-generators at Lijiaxia hydropower Station are made using the proposed method. Evaluation results indicate that in terms of daily operation, the energy saving of the evaporative cooling hydro-generator can be more than 300 tons standard coal equivalent per year as compared with that of air cooling generator. In terms of maintenance, the evaporative cooling method can save more than 5000 tons of standard coal equivalent per year. The comprehensive evaluation results show that the evaporative cooling method is significantly better than the air cooling. It can be seen that the proposed evaluation method may quantitatively calculate the merits of hydro-generator caused by cooling method, which provides guidance to select and improve cooling method of hydro-generator.