This paper presents a method to derive optimal viewpoints for flying robot based transmission tower inspection, which applies point cloud model. A safe envelope is established according to safe transmission tower inspection rules. Essential inspection factors are proposed to evaluate the quality of candidate viewpoints, which includes visibility, dimensionality feature of point cloud as well as the distance between viewpoint and transmission power. A score function is constructed to quantify candidate viewpoints, while multiple attribute decision theory is applied to calculate the weight of each factor. Particle swarm optimization(PSO) is used to find the optimal viewpoints set. Both simulation experiment and practical observations are carried out. The results prove that optimal viewpoints have a great contribution for accurate transmission tower inspection. Final results are compared to patch-based method and proved to be feasible.