Sag of transmission line is the key constraint factor for increasing transmission capacity for transmission line. Traditional sag monitoring methods which have relatively complex calculation models with environmental sensitive parameterscannot meet the requirements of dynamic transmission capacity increase.So, the paper in-depth study of relationship among line traveling wave transmission time differences, line length and sag, a novel transmission line sag calculation model is proposed. And designed a new line of dynamic capacity increase program.Real-time monitoring of environmental parameters and line sag and other operating parameters is based on Traveling Wave Fault Location Devices(TWFLDs) and micro weather station,Power Dispatching Center calculated and analyzed to determine the maximum allowable ampacity of the line,in order to achieve dynamic capacity of transmission lines.