The principle of the method based on reflectance phase for calculating the Q_(ext) of cavities is analyzed and the differences between the ideal coupling circuit used in the method and the practical equivalent circuit are pointed out in this paper.Firstly,the transmission line couples with a cavity by only partial inductance in the practical circuit.Secondly,there are coupling slots in π mode double-gap cavity.These two differences will bring on failure of the method in calculating the Q_(ext) of cavities.Q...