The loss of excitation protection action occurred during generator leading phase operation test, this paper makes a detailed analysis on the loss of excitation protection parameters setting calculation process, the determination and check of leading phase test condition, and the low exciting limit parameters setting during tests. Then arrivals at the conclusions that the loss of excitation protection setting calculation method in the instructions of REG126 protection is error, the leading phase test condition check process is error, and the loss of excitation protection device itself has certain error. Finally, in order to avoid the similar incidents, puts forward prevention measures, it is recommended on production of ABB REG126 type protection proposed comprehensive verification of loss of excitation protection setting value, and put forward the voltage value and margin settings in leading phase test condition check.