Abstract: Under the action of earthquake, the large span transmission lines with supporting towers is a complicated system of spatial coupling vibration. It is necessary to determine the reasonable computation scheme of this system for engineering design.The conventional methods will never be valid to calculate the seismic responses of the large span transmission lines and their supporting towers under the condition of unknown rigidity distribution. In this paper, an approach with noverl idea is presented.The upper bound seismic responses can be determined by putting the product function specially defined in this paper in extrema. Only two modes which have contributions to the responses need to be considered in design. as all the other high modes have strictly no contributions.In thd paper, the mechanical model of tower-line coupling vibrstion is given in X-Y plane. We compare the unknown rigidity distribution with known rigidity distribution in 2-degrees-of-freedom(DOF). We find the upper bound seismic loads in unknown rigidity distribution are always greater than the seismic loads in known rigidity distribution.