Abstract: In order to improve the accuracy of relative position determination of a lunar explorer (lunar lander and rover), we designed the same-beam interferometry method according to the actual signals of the lander and the rover of CE-3. We analyzed the method for calibrating device internal delay using the strong radio source, gave the method for calculating the differential phase delay from group delay, and analyzed the accuracy of relative position determination by simulation. Finally, we used the same-beam interferometry to observe the rover and the lander of CE-3. As the results, using the lander's data-transmission signal and the rover data-transmission or telemetry signal, the differential phase delay data were obtained with a 1ps random error, and the accuracy of rover relative position was improved to about 1 m.