Abstract: Precise time transfer is one of the most fundamental works in the field of time frequency. GNSS has become an important means of precise time transfer due to its advantages. In particular, the time transfer technology based on high-precision carrier phase observations developed in recent years has become a research hotspot in the GNSS based time-frequency field. This study comprehensively summarizes the research on GNSS carrier phase precise time transfer, expatiates on the observation model and ambiguity resolution method, and discusses the key technical issues such as precision, consistency, robustness, continuity, real-time, and integrity which are involved in the data processing. It is pointed out that this field should focus on the unity of un-differenced and differenced processing models, the unity of time service by integrating different mechanisms, and the seamless time service of sky, earth, sea and underground.