In order to solve the problem of the trajectory planning of robotic plasma cladding for complex surface coating, the concept of NURBS function open/closed-loop mapping is proposed in this work to explore the new approach of the trajectory planning for complex NURBS surfaces. The trajectory planning is carried out by a 2D-NURBS curve C(u) on a 2D plane, which is mapped on a predefined 3D-NURBS surface S(u,v) using the NURBS surface function to form a 3D mapping curve named NURBS function mapping (NURBS-FM) curve. Using the fixed step (FS) interpolation, the equal chord length (ECL) interpolation, the equal arc length (EAL) interpolation and the equal bow height (EBH) interpolation, etc., a complex curve can be interpolated. The FS/ECL/EAL/EBH can be defined as constraint. Depending on where the constraint is applied, the NURBS-FM can be divided into open-loop mapping and closed-loop mapping. The NURBS function open-loop mapping (NURBS-FOLM) is carried out along the route of u→C(u)→S(u,v), while the NURBS function closed-loop mapping (NURBS-FCLM) along the route of u→C(u)→S(u,v) →u →…. The constraint is applied to the 2D-NURBS curve in NURBS-FOLM, while to the 3D NURBS-FM curve in NURBS-FCLM. The NURBS-FOLM and NURBS-FCLM can ensure that the interpolation points on the 2D-NURBS curve and 3D NURBS-FM curve have the ECL/EAL/EBH characteristics, respectively. When the 3D NURBS-FM curve is regarded as plasma cladding trajectory, the NURBS-FCLM can provide engineers with a new ECL/EAL/EBH interpolation method for cladding trajectory. The simulation and experiment verify the NURBS-FCLM is feasible and effective. With the increasing application of complex surface protective coatings prepared by plasma cladding, the NURBS-FCLM has broad application prospects inevitably.