In order to solve the limitations of traditional manual teaching methods in industrial robot spraying operations, such as time consuming, error-prone and relying too much on manual labor，a trajectory planning method for surface spraying was proposed. In this method, the surface to be machined was parameterized to a plane, the offset curves were generated on the plane, and then the offset curves were inversely mapped to a 3D surface; the offset curve length and spray opening angle of the spray gun were calculated according to the grid model geodesic method, and the contour lines were generated in equal proportions on the offset curve; the least squares B-spline fitting curve was used to smooth the data, and particle swarm optimization algorithm was used to optimize the relationship among spraying speed, spray gun height and film thickness, and then the curved surface spraying trajectory was determined. The results show that the method is simple and practical, and can ensure the accuracy and feasibility of the curved surface spraying trajectory.