The invention discloses a multi-communication system compatibility numbering method which comprises the following steps: firstly, judging whether the system depth of a multi-communication system is increased after a new system element is added, if so, expanding the number of bits of a full-system number, and updating the full-system number of each communication terminal in a multi-communication system; if the number of the communication terminals is not increased, assigning a full-system number to each communication terminal in the new system element; during communication, judging whether communication between the communication terminals is in system element internal communication or cross-system element communication, if the communication is in system element internal communication, carrying out communication according to an encoding mode in the system element, and if the communication is in cross-system element communication, writing the number of the whole system into communicationcodes of the two communication terminals which communicate with each other. Thus, by setting the full-system codes according to the system depth change of the multi-communication system, the terminalID conflict of the multi-communication system in the bottom-to-top expansion mode is avoided.