The excitation mode is one important component part of synchronous generator. Its major objective is to maintain generator terminal voltage and improve power system stability. If there are low technical performance and automation to the excitation, it will cause enormous hidden danger on the operation of unit. In order to satisfy the high standard of generator quality for the power system needs，advanced excitation system should be adopted, and so it can ensure that the power supply unit shall be done in the safe and stable condition at long time. This paper mainly introduces the different excitation modes of 600MW or above power generator unit. With a further discussion on characteristics of different types of excitation mode, this paper suggests potential source brushless excitation mode should be adopted in great capacity generator.