A method for sending a trigger message, wherein the method comprises: when a third-party application server, AS, needs to establish a connection with machine-to-machine communications user equipment, M2M UE, sending, by the third-party AS, a trigger message to a device trigger application server, DT-AS, wherein: the trigger message comprises at least an identifier of the machine-to-machine communications user equipment M2M UE, so that the DT-AS determines, according to the identifier of the M2M UE, whether the M2M UE has registered with the DT-AS, and if the M2M UE has registered with the DT-AS, the DT-AS sends the trigger message to the M2M UE, receives a message that is sent by the M2M UE in response to the trigger message, and sends, to the third-party AS, the message in response to the trigger message; and receiving, by the third-party AS, the message that is sent by the DT-AS in response to the trigger message; wherein: the M2M UE is connected to a network at which the DT-AS is located, the DT-AS is a server in a 3GPP network, and the third-party AS is an M2M server outside the 3GPP network, wherein before the sending, by the third-party AS, a trigger message to the DT-AS, the method further comprises: acquiring, by the third-party AS, an IP address of the DT-AS and/or a port number of the DT-AS; and setting an IP address of the third-party AS and/or the port number of the DT-AS that are/is in the trigger message to a preset value or values, wherein the preset value or values is/are used by the DT-AS to identify the trigger message; or setting the IP address and/or the port number of the DT-AS that are/is in the trigger message to a preset IP address and/or a preset port number respectively, said preset IP address and/or preset port number having been agreed in advance by the DT-AS and the third-party AS, so that when a message is received by using the preset IP address and/or the preset port number, the DT-AS can identify the message as the trigger message. when a third-party application server, AS, needs to establish a connection with machine-to-machine communications user equipment, M2M UE, sending, by the third-party AS, a trigger message to a device trigger application server, DT-AS, wherein: the trigger message comprises at least an identifier of the machine-to-machine communications user equipment M2M UE, so that the DT-AS determines, according to the identifier of the M2M UE, whether the M2M UE has registered with the DT-AS, and if the M2M UE has registered with the DT-AS, the DT-AS sends the trigger message to the M2M UE, receives a message that is sent by the M2M UE in response to the trigger message, and sends, to the third-party AS, the message in response to the trigger message; and receiving, by the third-party AS, the message that is sent by the DT-AS in response to the trigger message; wherein: the trigger message comprises at least an identifier of the machine-to-machine communications user equipment M2M UE, so that the DT-AS determines, according to the identifier of the M2M UE, whether the M2M UE has registered with the DT-AS, and if the M2M UE has registered with the DT-AS, the DT-AS sends the trigger message to the M2M UE, receives a message that is sent by the M2M UE in response to the trigger message, and sends, to the third-party AS, the message in response to the trigger message; and receiving, by the third-party AS, the message that is sent by the DT-AS in response to the trigger message; wherein: the M2M UE is connected to a network at which the DT-AS is located, the DT-AS is a server in a 3GPP network, and the third-party AS is an M2M server outside the 3GPP network, wherein before the sending, by the third-party AS, a trigger message to the DT-AS, the method further comprises: acquiring, by the third-party AS, an IP address of the DT-AS and/or a port number of the DT-AS; and setting an IP address of the third-party AS and/or the port number of the DT-AS that are/is in the trigger message to a preset value or values, wherein the preset value or values is/are used by the DT-AS to identify the trigger message; or acquiring, by the third-party AS, an IP address of the DT-AS and/or a port number of the DT-AS; and setting an IP address of the third-party AS and/or the port number of the DT-AS that are/is in the trigger message to a preset value or values, wherein the preset value or values is/are used by the DT-AS to identify the trigger message; or setting the IP address and/or the port number of the DT-AS that are/is in the trigger message to a preset IP address and/or a preset port number respectively, said preset IP address and/or preset port number having been agreed in advance by the DT-AS and the third-party AS, so that when a message is received by using the preset IP address and/or the preset port number, the DT-AS can identify the message as the trigger message.