3D digital design is a popular research issue in the design of transmission lines in recent years. This paper studies a transmission line path planning method based on 3D digital design. Firstly the cloud platform and three-dimensional GIS technology are used to carry out the digital modeling of complex geographic information, and then the digital model is equivalent to a weighted raster matrix. At last, the Q-learning method is introduced for optimal path planning. The experimental results show that through the deep mining of 3D data, the method studied in this paper can increase the design efficiency by 65% and reduce the design cost by 5%.