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Group delay is a key parameter describing the phase characteristic of the signal transmission system, which can also measure the phase linearity of the continuous signal passing through transmission system. The group delay of Frequency-Translating Devices (FTD) is measured by the double-frequency phase difference method in this paper. But the measurement error will increase in the FTD whose group delay changes greatly. In view of this problem, an optimization method is put forward. Combined with polynomial fitting of high order, this proposed method, which uses the reasonable selection of the parameters such as measuring aperture and measurement interval, can improve the measurement precision and the test efficiency.