The utility model discloses a register method for user based on SIP, comprising a terminal transmission register request; a terminal receiving message, when the message indicates that user number has been registered at other terminals and determines to replace the register of other terminals, the sending message performs register for transmission register request terminal and cancels the registerof other terminals, other terminals are stopped to transmit register request message to the register server. The utility model also discloses another method based on SIP which comprises a receiving register request message; authorization is needed, when the user numbers are found to pass other terminals and establish register, and then transmit message to the terminal, when receiving request for replacing message, register is performed for the transmission request terminal, the registers of other terminals are cancelled and message is transmitted to indicate that other terminals are cancelled. The utility model also discloses a register system of user and corresponding terminal, and register server. The utility model has the advantages that, the user can register on other terminals when the previous register is not cancelled.