The invention relates to an implementation method for assembling an SDAP head by a terminal in a 5G system, and belongs to the technical field of mobile communication. The implementation method for assembling an SDAP head by a terminal in the 5G system is characterized by comprising a process of sending uplink data by a terminal PDCP sub-layer and a process of receiving downlink data by the terminal PDCP sub-layer. According to the method, the header addition and deletion operations of the SDAP sub-layer are moved from the SDAP sub-layer to the PDCP sub-layer for processing. In this way, the data processing efficiency of the PDCP sub-layer of the terminal in the 5G NR system is improved, and meanwhile related modules of the PDCP sub-layer in 4G can be conveniently reused.