In order to fulfill the requirements of high-speed and real-time down data transmission of unmanned aerial vehicle data link, a high-speed data transmission unit based on PowerPC processor and IEEE1394 bus is realized. First, the paper introduces the design requirements of down data transmission system in unmanned aerial vehicle data link. Then it describes the system architecture of down data transmission system and the realization of software and hardware in data transmission unit in detail. At last, the function and performance of data transmission unit are validated through the encirclement of IEEE1394 bus. The experiment indicates that the data transmission unit has the characteristics of simple design, high transmission capability, high reliability and low power dissipation, which has broad application prospects.