The invention discloses a power flow calculation method and system based on an electro-thermal coupling effect of a transmission line. The method comprises the steps of acquiring meteorological information and information of all branches and nodes in an electric power system, and setting initial values of the voltage amplitude and phase angle of each node to be calculated and the resistance of each transmission line in the power flow calculation; calculating an active node admittance matrix and a reactive node admittance matrix of the electric power system according to the information of allbranches and nodes in the electric power system; correcting the node injection active power, and iteratively calculating the voltage phase angle of a non-reference node; correcting the node injectionreactive power, and iteratively calculating the voltage amplitude of a PQ node; iteratively calculating the resistance of the transmission line according to the meteorological condition; and judging whether a convergence condition is satisfied or not. The method solves a problem of calculation errors generated by neglecting the electro-thermal coupling effect of the transmission line in the conventional power flow calculation, and improves the calculation efficiency.