In order to improve the transmission capacity and the dynamic capacity increase of transmission line, the radial temperature rise phenomenon of transmission line is studied and analyzed. In this paper, the overhead transmission line is taken as an example. Firstly, the theoretical value of the conductor temperature is calculated based on the heat balance equation. Then, the electromagnetic coupling finite element stranding model of the transmission line temperature field is established to calculate the radial temperature distribution of the transmission line and study the influence of different factors on it. Finally, the effect of dynamic capacity expansion of the transmission line is analyzed based on the analysis results of the temperature field, and the minimum current carrying capacity is calculated according to the temperature distribution of the conductor. The results show that the radial temperature distribution of the transmission line is not uniform. The internal temperature is high and the surface temperature is low; the temperature of the transmission line is affected by different current, wind speed, ambient temperature and time. The radial temperature difference can generally reach 0.58~4.53℃, so the radial temperature difference of the overhead conductor is studied. According to the temperature distribution of the wire, the minimum current carrying capacity is calculated, which is beneficial to improve the dynamic capacity of the transmission line so as to ensure the safe operation of the line.