The dynamic line rating technology can be used to substantially improve the transmission capacity of overhead lines

and decrease or delay re-building transmission lines. This paper analyzed the force balance of overhead line considering wind speed and direction in a deflection plane. On this basis

a mechanics analysis model of line’s tension and sags was established. The state equation of over-head line was corrected using the relation of temperature and sags

and the ultimate temperature of over-line was deduced. The advantages and drawbacks of some mathematic models used to calculate transmission capacity were analyzed. In order that the DLR system obtains the optimum dynamic capacity curve

a model combining the weather model（WM） with temperature-sag（TS） model was adopted. Simulation test verified validity and practicability of this model. Finally

the necessity and significance to use DLR in Chinese power grid were presented.