A protocol stack software system of mobile terminal includes common user identification module management module, user interface module, non-access layer and access layer, wherein the non-access layer comprises a mobility management module for performing mobility management of the mobile terminal, characterized in that, the non-access layer also includes a network search module for controlling the selection of mobile communication network and controlling the register of the mobile terminal with the selected mobile communication network.; The protocol stack software system of the present invention separates network search function of the non-access layer, makes the structure of the non-access layer more clear, therein the network search module is special for controlling network search and cell residence procedure, while the mobility management module is for the management of the mobility and taking the responsibility for maintaining the connection of upper layer service. The present invention also provides a method of searching mobile communication network by mobile terminal having the above protocol stack software system.