Aimed at the application limitations of TM, TC, AOS, Proximity-1 space data link protocol, this paper analyzes the unified space data link protocol specification standardized by CCSDS, and introduces the design drivers of USLP transfer frame primary header and data field, as well as the differences with other space data link protocol. This protocol enables service data units to be multiplexed together in one channel and provides service types for a physical channel, master channel, virtual channel, multiplexer access point channel. This paper shows logical relationships among data transfer service, channel multiplexing, channel identifier and service data unit. Transfer telemetry and command data over a space link , for instance, the applicability and recommended of USLP are analyzed.