Abstract: To obtain a high precision relative measurement between the orbiter and the ascender of the CE-5 lunar probe, a same-beam VLBI algorithm is propsed in the framework of the deep-space interferometry processing center. The conditions and results of getting a right cycle ambiguity are discussed. After introducing the group-delay-aided phase interferometry,the random errors of interferometrical delay are suppressed greatly. It provides a priori condition for a right cycle ambiguity. Finally,the proposed algorithm is verified by using the CE-5 data,laying the foundation for the X-band same-beam VLBI in the CE-5 task.