Both economy and reliability should be considered in transmission planning， while the environment in the planned area directly affects the cost of the transmission line. This paper analyzes the relationship between the lines and the geographical distribution of the environment in the transmission planning， and proposes a path decision-making method with the environmental factors taken into consideration. On a raster map， the environment costs through each cell are ranked by pattern recognition， on the basis of which an optimization model for the transmission lines routing considering environment costs is developed. In the light of the characteristics of the optimization problem， a two-stock smart ant algorithm is proposed to solve this problem. The method proposed is proved correct and efficient with a case study.