Aiming at the on-orbit engineering problem of about 286× rate mismatch between high-speed image acquisition output via 33 MHz SPI and low-speed RS422 downlink of space scientific experimental payloads， this paper designs and implements a mailbox-type credit flow control low-speed image downlink transmission system adapted to aerospace resource constraints and capable of link fault self-recovery for space friction and wear test boxes. The system adopts a collaborative interaction architecture composed of data packets and residual space query credit flow control packets. The FPGA main controller for experimental control constructs a three-stage pipeline processing architecture including SPI interface control， application data mailbox multiplexing and application data downlink organization， and the imaging terminal based on RK3588 realizes the coordinated sending strategy of data stream burst and credit flow control. A standardized mathematical model of credit flow control is established， and the constraint conditions for buffer overflow prevention are fully derived. A dual high-reliability guarantee mechanism consisting of heartbeat continuation packets and timeout-triggered active transmission at the transmitting terminal is developed to solve transmission interruption caused by loss or merging of flow control packets due to on-orbit link noise and thread scheduling delay. The experiment results show that the transmission of a single 256-byte SPI packet takes 92 μs， the steady-state interaction frequency of transmission and flow control is about 25 times per second， and it takes approximately 10 minutes to fully transmit a 3 MB single-frame image， with no buffer overflow or transmission deadlock in the whole process. This scheme has been applied on orbit to the space friction and wear test box， which can effectively adapt to the huge rate gap between high-speed writing and low-speed reading of space payloads， and provide complete engineering reference for embedded data transmission systems with mismatched transmission rates in aerospace， industrial control and other fields.