To ensure the security and stability of the power grid after implementing dynamic capacity-increase for overhead transmission line, a new dynamic capacity-increase method based on congestion management analysis is proposed. The reasons for transmission capability restriction are clarified based on ultra-short-term forecasting data, and the key transmission lines are identified by congestion management analysis considering dynamic security constraints. According to the congested transmission lines and capacity-increase periods, the permissive current-carrying capacity in a future period is calculated based on weather forecasting data. The transient process of temperature rising is considered in the post-fault permissive current-carrying capacity calculation for the lines whose congestion time is within certain range. On this basis, the amount of capacity-increase is determined to meet the stability and security constraints. Based on the above scheme, a dynamic capacity-increase system for overhead transmission line is developed.