Abstract: Very long baseline interferometry (VLBI) uses phase calibration (PCAL) signal to calibrate link delay. However, the existing PCAL signal can only obtain the relative change of link delay. To meet the requirements of accurate measurement of absolute UT1, station clock error compensation, maintenance and diagnosis, and to improve the accuracy of deep space TT&C in the future, absolute link delay is necessary to know. In actual link, it is difficult to measure the absolute delay due to the existence of converter and other devices. This paper proposes a method to measure the absolute link delay using PCAL based on the linear variation of comb spectrum phase with frequency. At the same time, this paper designs a lightweight PCAL extraction software for fast diagnosis and absolute time delay measuring, which has been applied in practical work.