The ampacity is one of the most important parameters to ensure the safe operation of overhead transmission lines. According to the characteristics of heat generation and dissipation of the overhead transmission lines, a 2D finite element model of overhead electrical conductor considering the coupling of electromagnetic field and thermal field is established. Based on corresponding electromagnetic-thermal boundary conditions, electromagnetic field and temperature field distributions of overhead electrical conductor are obtained and the Joule's loss, as heat source, calculated in electromagnetic field analysis is coupled with temperature field. The numerical iterative method is applied to calculate the ampacity of overhead transmission lines. Moreover, the influence of different environmental factors on ampacity of overhead transmission line is investigated based on this simulation model. The research has some reference significance to the thermal analysis and ampacity calculation of overhead transmission lines.