The embodiment of the invention discloses a method, a device and a system for allocating and querying service gateways, wherein the implementation of the method for allocating the service gateways comprises the following steps: acquiring information on tracking areas and on the service gateways corresponding to the tracking areas; allocating the corresponding relationship between the tracking area identifications of the tracking areas after being processed by mask codes and the service gateways, wherein the tracking identifications corresponding to the same service gateway are the same after being processed by the mask codes; transmitting the corresponding relationship between the tracking area identifications after being processed by the mask codes and the service gateways to a domain name system server; and transmitting a rule of mask code processing to a mobile management entity or the domain name system server. Because the TAIs (tracking area identifications) of the same service gateway are the same after being processed by the mask codes, the TAIs of the same service gateway can be combined into one so as to reduce the quantity of the TAIs and reduce the corresponding relationship table entries of the TAIs and SGW information on the DNS (domain name system server).