A distributed cluster processing system, comprising at least one external interface unit (302a, 302b, 302c, 302d), multiple processing units (304a, 304b, 304c, 304d), and at least one switching unit, wherein: each of the at least one external interface unit (302a, 302b, 302c, 302d) is connected between a corresponding processing unit of the multiple processing units (304a, 304b, 304c, 304d) and an external network element, and is configured to receive a packet from the external network element, forward the received packet to the processing unit directly connected to the external interface unit (302a, 302b, 302c, 302d), and send a processed packet to the external network element; each of the multiple processing units (304a, 304b, 304c, 304d) is configured to perform specified service processing and is respectively connected to the switching unit, so that the multiple processing units (304a, 304b, 304c, 304d) and the switching unit form a star topology structure; each of the multiple processing units (304a, 304b, 304c, 304d) is further configured to receive a packet from the switching unit or the packet from the directly connected external interface unit, process the packet, determine lower-level service processing and determine a lower-level processing unit according a session processing policy, add a tag containing information about the lower-level processing unit in the processed packet and send the processed packet to the switching unit, and if it is determined that all processing on the packet has been completed, send the packet to one of the at least one external interface unit (302a, 302b, 302c, 302d); and the switching unit is configured to receive packets from the multiple processing units (304a, 304b, 304c, 304d), and forward, according to the tag containing the information about the lower-level processing unit and added in the received packets, the packets to a processing unit among the multiple processing units (304a, 304b, 304c, 304d) that is indicated by the information about the lower-level processing unit. each of the at least one external interface unit (302a, 302b, 302c, 302d) is connected between a corresponding processing unit of the multiple processing units (304a, 304b, 304c, 304d) and an external network element, and is configured to receive a packet from the external network element, forward the received packet to the processing unit directly connected to the external interface unit (302a, 302b, 302c, 302d), and send a processed packet to the external network element; each of the multiple processing units (304a, 304b, 304c, 304d) is configured to perform specified service processing and is respectively connected to the switching unit, so that the multiple processing units (304a, 304b, 304c, 304d) and the switching unit form a star topology structure; each of the multiple processing units (304a, 304b, 304c, 304d) is further configured to receive a packet from the switching unit or the packet from the directly connected external interface unit, process the packet, determine lower-level service processing and determine a lower-level processing unit according a session processing policy, add a tag containing information about the lower-level processing unit in the processed packet and send the processed packet to the switching unit, and if it is determined that all processing on the packet has been completed, send the packet to one of the at least one external interface unit (302a, 302b, 302c, 302d); and the switching unit is configured to receive packets from the multiple processing units (304a, 304b, 304c, 304d), and forward, according to the tag containing the information about the lower-level processing unit and added in the received packets, the packets to a processing unit among the multiple processing units (304a, 304b, 304c, 304d) that is indicated by the information about the lower-level processing unit.