The invention provides a filling shape generating method for 3D printing, and the method is applied to the technical field of 3D printing. The method comprises the steps that outline ring data of each layer of a model are guided in; two or more parallel straight lines are generated in an outline ring; line segments in the outline ring are found; every two adjacent straight lines form one straight line group, and a filling waveform is generated in a reciprocating manner; and the straight line generating the filling waveform serves as a base line, whether the straight lines adjacent to the base line generate the filling waveform or not is judged, if the straight lines adjacent to the base line do not generate the filling waveform, the base line and the adjacent straight lines form a straight line group, a new filling waveform is generated, and the adjacent filling waveforms are the same or reverse in phase. According to the provided filling shape generating method for 3D printing, an adjoining plane between filling materials is increased, the mechanical strength and impact resistance of products are greatly improved, and the printed product is firmer and more durable.