There are many problems in the secondary wiring and logic design of GE generator-transformer unit protection since the four units successively putting into operation in Huaneng Yuhuan Power Plant. It is analyzed and improved about the generator-transformer unit protection, such as starting fast cut back of auxiliary power, zero-power triping of the line section, overexcitation protection of generator, secondary circuit wiring and logic design. It is shown that the protection functions are more perfect, effectively ensuring the safety of power system.