Abstract: The transmission-line theory based fast algorithm for voltage distribution along the ultra high voltage（UHV） transmission line with series capacitors（SCs） was proposed.Firstly,the two-port transmission parameter matrix of UHV transmission lines is transformed to the node admittance matrix,therefore,the lumped parameters of strict equivalent P model is obtained.Secondly,the parameters are filled into the commercial power flow software to calculate the power flow of the target grid which includes the UHV transmission lines with series capacitors.Thirdly,the power flow calculation results are regarded as the terminal conditions of the UHV lines with series capacitor,and the transmission parameter matrix is used to calculate the voltage distribution along the line rapidly.In the algorithm,the voltage distribution is obtained accurately;moreover,the reactive power supply capacities of both sides can be taken into account,which implies that the engineering applicability is very strong.At last,the relative disposition scheme of series capacitors and line shunt reactors as well as the centralized/distribution layout scheme of UHV series capacitors were investigated,and it was concluded that the ’shunt reactor at SC bus side’ scheme and the distributed layout scheme of UHV SCs are incident to suppress the fundamental frequency steady overvoltage,compared with the ’shunt reactor at SC line side’ scheme and the centralized layout scheme of UHV SCs.According to the proposed algorithm and the conclusions,the guidance suggestion was given for the planned UHV transmission projects.