The transmission characteristics of ultra-high voltage (UHV) transmission line are analyzed. The influence of harmonic order for harmonic transfer coefficient, as well as the length of the line are summarized. With the results researched, we know that the harmonic transfer coefficient is not linear changed with the increase of harmonic order and line length. As different harmonic orders, it is with cyclical changes to be amplified or attenuated along with changes in transmission length because of stand wave effect.