针对空间科学实验载荷高速图像采集输出（33 MHz SPI）与低速 RS422 下行链路存在约 286 倍速率失配的在轨工程难题，面向空间摩擦磨损实验箱设计并实现了一套适配航天资源约束、具备链路故障自恢复能力的邮箱式信用流控低速图像下行传输系统。系统采用数据流包、剩余空间查询信用流控包协同交互架构；实验控制FPGA主控构建SPI接口控制、应用数据邮箱复接、应用数据下行组织三级流水处理架构，成像RK3588端实现数据流突发与信用流控协同发送策略。建立标准化信用流控数学模型，完整推导缓存防溢出约束条件；针对空间在轨链路噪声、线程调度延迟引发流控包丢失、合并导致传输中断问题，设计了心跳续流包、发送端超时主动发送双重高可靠保障机制。实验结果表明：单次SPI发送256字节耗时约为92 μs，稳态发送-流控交互频率约为25 次/s，完整传输一帧 3 MB图像耗时约10 min，全流程缓存无溢出、无传输死锁。该方案已在轨应用于空间摩擦磨损性能实验箱，有效适配空间载荷高速写入与低速读出的巨大速率差，为航天、工业控制等多速率不匹配嵌入式数据传输系统提供了完整工程参考。

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.