The accuracy of DC current transformer (DCCT) is an important basis for its state evaluation and on-site acceptance. It plays an important role in the stable operation of HVDC protection and control. However, the field calibration is not successfully carried out by the factors such as the test method, the field calibration equipment and the key technology. Combined with the actual situation of converter station, this paper proposes a DCCT based asynchronous field synchronous calibration method based on absolute delay. Aiming at the field test of DCCT absolute delay, we develop a high-accuracy DC transformer field synchronous calibration system. In this paper, an uncertainty analysis model of the calibration system is built to ensure the reliability of the calibration results.The test method is applied and verified in the field test of DC current transformer of ±500kV Yongfu DC project, providing reference for the field test of DC current transformer of converter station in China.