A method for packet reassembly and reordering, comprising: receiving a cell sent by a source port, wherein the cell carries a Source Identification (SID), a packet sequence number and a cell sequence number; preprocessing the received cell according to the SID to determine whether the cell shall be inserted into a packet reassembly database; ordering cells in the packet reassembly database according to the packet sequence number to obtain a correctly ordered packet; if the correctly ordered packet is a complete packet, ordering the cells of the correctly ordered packet according to the cell sequence number to obtain correctly ordered cells; and performing a packet reassembly for the correctly ordered cells. Correspondingly, a network device and a communication system are provided.