For positioning of spacecraft with VLBI phase-reference, an extragalactic radio source was usually served as phase reference. After fringe fitting on the radio source, the solutions including delays and rates mainly from the propagation media were adapted to calibrate the observed phase of the spacecraft. The common phase errors of the nearby source pair were significantly reduced in their differential phases, allowing accurate relative position measurement. However, this traditional phase-reference method requires strong and nearby extragalactic radio source, which is hard to fulfill due to the lack of strong calibrators on the sky. Since the radio flux density from the spacecraft is much stronger than that of the extragalactic radio source, it is possible to use the spacecraft as phase reference. In this paper, we investigated the feasibility and advantages of this so called inverse phase reference method, based on real VLBI observations. We compared the data analysis results using the two phase reference methods, it proved that the inverse phase reference is capable to measure accurate relative positions, and could achieve better results that the traditional methods, especially when the extragalactic radio source was weak. planetary probe, VLBI phase reference, VLBI inverse phase reference, simulation of weak source