Abstract: To solve the problem of long transfer delay or low utilization of bandwidth in the reliable services in deep space file delivery protocols, this paper, based on the CCSDS file delivery protocol(CFDP)with the relationship that the file transfer delay shortens with the decrease of the length of protocol data unit(PDU), achieved the goals of reducing the transfer delay and keeping the bandwidth optimization simultaneously through optimizing the length of PDU at the stage of retransmission. The theoretical analysis and simulation results show that under the typical earth to mars communication scenario, the method presented in this paper, compared with CFDP, reduces the transfer delay by about 12% at the expense of an increase of about less than 1% in bandwidth, and compared with RSFDP, reduces the bandwidth by about 3% at the expense of an increase of about 15% in transfer delay.