Abstract: Due to the increasing uncertainties in power systems, the need for flexibility in the operation and planning of modern power systems is more urgent than ever. As the penetration of renewable energy sources increasing, the limit of network transmission flexibility in system flexibility supply and demand becomes more significant. The study of network transmission flexibility and its quantitative evaluation is the key to ensure the availability of flexible resources and to enhance the flexibility of power systems. This paper reviews and outlooks the research on power system network transmission flexibility and its evaluation. First, the definition of network transmission flexibility is introduced and its characteristics are outlined, including temporality, non-directionality, and inherency. Secondly, the research and methods of network transmission flexibility are discussed. Moreover, the assessment indexes of network transmission flexibility are sorted out from three aspects: network available transmission capacity, network safety transmission margin and network transmission flexibility index. Finally, the research on network transmission flexibility of power system is summarized and prospected, and then the issues that need attention in the research process are explained.