The invention discloses a method and a system for planning a multi-layer multi-pass welding path of a single bevel groove. The method comprise the following steps that modeling is carried out on the weld pass arrangement of a right bevel groove corner joint; an equal-height filling strategy is adopted for weld pass planning, the cross section of backing weld pass is fitted in a right angle triangle, the first weld passes of the second layer and every layers above are fitted in a right angle trapezoid, the last weld pass is fitted in trapezoid, and the rest weld passes are fitted in rhombus; the number of layers and passes of single bevel groove welding, the position of a welding gun, inclination angle of the welding gun and swing amplitude of the welding gun are determined; and path planning is carried out based on the determined welding gun posture to realize multi-layer multi-pass welding of single bevel groove. According to the method and system, the multi-layer multi-pass weldingpath planning model is established for the single bevel groove, the height of the backing weld pass and a filling weld pass need to be determined by a user, and the position, posture, swing amplitude, welding process parameters and the like of the welding gun can be calculated by using the model to get results.