In certain regions of dense forest or desert, deploying man-made ground control points (GCPs) and/or surveying them would be very hard. The paper proposes a novel algorithm to estimate the absolute phase offset for airborne dual-antenna interferometric SAR (InSAR) system without GCPs, which costs less to generate digital elevation model (DEM). The proposed algorithm is based on tie points, whose height is unique, in the overlap area of two different interferometric data acquisitions. The real airborne dual-antenna InSAR data in X-band from the Institute of Electronics, Chinese Academy of Sciences (IECAS), is processed to evaluate the performance of the proposed algorithm. And the phase offset values are obtained for both acquisitions with the proposed algorithm and the traditional phase offset estimation algorithm with GCPs. Meanwhile, compared with the traditional phase offset estimation algorithm with GCPs, the proposed algorithm has similar performance as shown by the results.