Flexibility assessment and planning have become the core and key of a power system which has a high proportion of renewable energy. The existing research on flexibility mainly focuses on the matching level of flexible supply and demand, which ignores the impact of line carrying capacity on flexible supply and the demand transmission process. In view of this, first, a flexible supply and demand transmission model of a power system is established by analyzing the transmission process of flexibility at the line level. Then, the concept and measurement index of flexible carrying capacity are proposed. On this basis, the evaluation method of supply and demand balance considering flexible carrying capacity is proposed. The analysis of the causes of the lack of flexibility and identification of the weak links of the system can be handled through this method. Finally, a two-stage expansion planning method based on flexibility assessment is proposed, and the validity and rationality of the method are verified by using the Garver-18 node system. The example shows that on the one hand, the flexible carrying capacity index can quantify the transmission capacity of power grid. This is more in line with the actual situation of power system. On the other hand, the proposed method can tap the potential of transmission lines, expand the flexible operation space of grid structure, and effectively improve the flexibility of the power system.