The utility model provides a multi-core-to-single-core conversion joint, which comprises a body, wherein both ends of the body are respectively provided with a multi-core male joint and a single-core female joint, a multi-core lead-in end is arranged at one end of the body, and comprises an insertion groove matched with the multi-core male joint, a positioning key is arranged on the multi-core lead-in end which is connected with the multi-core male joint in a positioning way through the positioning key, a single-core lead-out end is arranged at the other end of the body, and comprises an insertion needle and a case, one end of the insertion needle is communicated with a communication cable core arranged on the multi-core male joint through the multi-core lead-in end, the other end is connected with the insertion groove of the single-core female joint, one end of the case is connected with a connecting cable core matched with the communication cable core on the multi-core male joint through the multi-core lead-in end, and the other end is matched and connected with the single-core female joint case. The utility model adopts the mutually connected multi-core lead-in and single-core lead-out end, is respectively connected with the multi-core female and the single-core female joint, realizes the conversion between the multi-core joint and the single-core joint, does not need to simultaneously use the multi-core joint and the single-core joint in a logging instrument, reduces connecting wires, and improves the use stability of the logging instrument.