The invention discloses a 3D printing forming method for a composite part, and relates to a fast forming method of a composite part. According to the method, a method of alternately performing reinforcement layout and substrate spray deposition layer by layer is adopted, and defects of incomplete fiber impregnation, severe hole phenomenon, severe fiber deformation and the like in a traditional process method for preparing a long-fiber composite are overcome. The 3D printing and forming method adopts a layered manufacturing method, fiber cloth with uniform holes is taken as a reinforcement material, a spray deposition forming mode is adopted to deposit a substrate material on the reinforcement material, adhesion between the reinforcement materials is completed, and meanwhile, the processing means is adopted to remove an excess material synchronously; and with the adoption of the method, the composite part is formed rapidly, and the forming efficiency and quality are greatly improved.