In order to speed up the spread and withdrawal of onboard TT&C equipment, and to enhance the flexibility of onboard equipment, this paper puts forward a new calibration method by use of Offset-antenna which can simplify the architecture of onboard TT&C system. The new method makes use of the downlink carrier-frequency from the Offset-antenna as the signal-source, and adjusts the phase dispersion between sum channel and difference channel, so that the error voltage can indicate the position of the target. The experiment data shows that the result of the new method has good consistency and stability, and can meet the requirement, and also is consistent with the result of traditional method. So the new method can be used in Onboard TT&C system.