Baihetan hydropower station has 14 hydrogenerators with the unit capacity of 1000MW. On a global scale, there are no experiences on design and manufacture of such huge hydrogenerators. There are a lot of technical problems to be solved, among which the selection of cooling mode is a very key problem. Firstly, this paper put forward the general electromagnetic scheme of the air cooling and evaporative cooling hydrogenerators. Based on it, we carried out the simulation of the electromagnetic field and got the distribution of the heat source. Secondly, we carried out the simulation of the 3-D temperature field for the stator of generator with two kinds of cooling modes respectively. Finally, we made comprehensive characteristic comparison between these two cooling methods from the aspects of unit size, material consumption, the quantity of coolant usage and thermal performance, and the comparison provides valuable reference for the future selection of cooling mode in the Baihetan unit bid.