1 000 kV AC transmission and 500 kV AC transmission have different applicable transmission capacity and range. According to the different situation of power station, there should be a reasonable choice of voltage level, matching the corresponding circuits, series compensation degree, switch station. The traditional method uses the BPA simulation software to obtain that by continuous power flow algorithm. But this method is complicated and time-consuming. Calculation is particularly large when electricity has a variety of power demand analysis. This paper presents an algorithm based on the AC line transmission capacity, Matlab software programming is used to achieve the establishment of UHV AC transmission model and economical optimization strategy. The method can use thermal limit, voltage limit, landing stability constraints to get the AC line transmission capacity curve by considering the factor of power network strength. On the basis of this method, an UHV AC transmission model can be built combined with unit capacity annual cost algorithm, which can meet the power transmission of arbitrary transmission capacity and the optimal economic of any transmission distance, has an important reference significance and practical value for the planning and construction of electric transmission lines in China.