The invention discloses a planning method for the smooth trajectory of a robot in Cartesian space. The planning method comprises the following process: planning the positions and postures of single-section trajectories, such as a spatial line and an arc; carrying out planning on the smooth transition of a path; controlling the size of the transition region of the path by adopting a transition grade; fitting the path in the transition region by adopting a parabola, and fitting the postures by adopting a quaternion algorithm; and adopting a cycloidal acceleration and deceleration planning method containing acceleration and an acceleration restraint to carry out planning on the spatial trajectory of the path so as to obtain the smooth spatial position and posture trajectory of a robot. The method can effectively solve robot trajectory problems, such as pause, non-smoothness, dithering, path deviation and speed excess and improve the running stability of the robot. The planning method for the smooth trajectory, related to the invention, has certain flexibility, and not only can plan the positions and the postures in the Cartesian space but also can adapt to various robot structures, containing common 6-degree-of-freedom vertical joint mechanical arms, low-degree-of-freedom robots and redundant robots.