In order to solve the problem of ultra-high speed space transmission and ensure the realization of our country's space-based broadband communication network, the necessity of applying digital coherence scheme to ultra-high speed space laser communication is analyzed in this paper, four problems in achieving this goal existed at current stage are summarized as follows: single-mode fiber coupling, low noise amplification of the erbium doped fiber amplifier(EDFA), algorithm of digital encoding and decoding, and hardware architecture. The solutions to these four problems are put forward one by one: fast steering mirror (FSM) tracking and adaptive nutation, segmented noise suppression, floating point parallel processing algorithm and integration. This paper analyzes the development trend of space laser communication technology, and finally gives the prediction of the future development direction of technology.