The transmission capacity of overhead line is generally calculated under the assumption that the surrounding environment of the transmission line is constant, and it is quite conservative. In order to make full use of the transmission capacity of the existing overhead transmission lines, a new method of on-line dynamic capacity-increase is developed based on BOTDR temperature monitoring technology. By using the Brillouin optical time domain reflectometer (BOTDR) temperature monitoring technology on OPGW, distributed measurement can be done for the ambient temperature of the long distance overhead line in real time. The wire temperature is deduced by Newton’s iteration method. The real time load flow is calculated by substituting the choke point of temperature of real-time wire into the new simplified heat balance equation. The result shows that this new method can effectively improve the carrying capacity of the line and is of great theoretical value to design optimization of the line load flow. This work is supported by National Science and Technology Support Program of China (No. 2013BAA02B01).