Combined with the data delivery mechanism and queuing model of bundle protocol/Licklider transmission protocol (BP/LTP) in delay tolerant network (DTN), the more accurate retransmission round number is calculated, and the average length model of the custody queue of the relay node memory space is constructed. The model is used to measure the change trend of node memory space, and the joint optimization scheme is proposed based on the number of protocol data unit bundles and the length of Licklider transmission protocol (LTP) data segments. Theoretical analysis and simulation results show that, without affecting the normal data transmission, this optimization scheme can minimize the memory queue length of the node memory space, that is the smallest memory space. Under the premise that the arrival rate of bundle and channel bit error rate change, the optimized queue length reduces by 70.9% and 61.8%.