Electro-thermal coordination theory is important for the effective on-line utilization of the load carrying capacity of transmission components. However， the close coupling between electricity and heat complicates the power flow calculation. Aimed to solve this problem， the dynamic heat balance equation is introduced into power flow calculation model. Then a new power flow calculation model in which temperature is considered as a state variable by using implicit trapezoid discretization method is proposed， and the power flow calculation model considering the coupling of electricity and heat is formed. In addition， based on the deduced correction equation of Newton method a simplified algorithm which adapts to the electricity-heat coupling problem is proposed. The uniform transaction of electrical variables and temperature is realized by this algorithm. It provides not only the electrical information of power flow calculation， but also the thermal inertia process. Consequently， the carrying capacity of a conductor can be determined by its temperature. Finally， a simulation is carried to demonstrate the validity of the model and algorithm proposed in this paper.