The present invention discloses an identification number numbering method and a multipoint communication system. The identification number numbering method includes the following steps: sending an identification number packet to a multipoint communication bus by a master device; receiving the identification number packet via the multipoint communication bus, and temporarily storing an identification number according to the identification number packet by a first slave device; changing a voltage level of a master device control output pin of the master device; and when the first slave device determines that a voltage level of a first control input pin coupled to the master device control output pin is correspondingly changed, updating a first slave device identification number of the first slave device according to the identification number.