A packet data transmitting method for packet service domain is disclosed. After receiving a Service Data Unit (SDU), a Packet Data Convergence Protocol (PDCP) entity determines how to handle the SDU in a cache queue and process a Protocol Data Unit (PDU) record in a Serial Number (SN) window based on whether or not the cache queue includes any SDU and the SN window is full, respectively. When both the cache queue and the SN window are full, the PDCP entity processes PDU records in the SN window according to their respective weight values and processes an SDU in the cache queue into a new PDU accordingly. A packet data transmitting system for packet service domain is also disclosed, which can prolong the time window for confirming a PDU record in the SN window.