Abstract: A trajectory optimization method of two cooperative welding robot arms is proposed. Based on Frenet-Serret vector theory, a revised coordinate system for welding discrete point of complicated curve is established. The feasible region of welding start point is determined in an effective workspace for two cooperative robot arms, with the best welding position, constant welding speed, collision-free and torch working posture. The start point of welding is further optimized by minimizing the total angular displacement and the corresponding optimal welding path is obtained. An experiment is set up to compare the welding trajectory of the optimal start point with those of other feasible points. With analysis in each joint's average angular velocity, the volatility and peak of the angular velocity and total welding time, it is verified effective for the trajectory optimization method.