Starch composite gels can be used for three-dimensional (3D) printing aiming at producing food products with a designed shape. Sodium alginate (SA), as a common thickener, is used for the fabrication of starch composite gels in 3D printing protocols. However, the printing windows and mechanism of 3D printing of starch and SA composition gel remain unclear. The geometric accuracy, rheological properties, small-angle X-ray scattering (SAXS) and scanning electron microscopy (SEM) were used to evaluate starch composite gels and their products to get the structure-properties-printing ability and accuracy. Computational fluid dynamics (CFD) simulations were employed to simulate the 3D printing process. The results showed that SA was capable of improving the 3D printing feasibility of the starch gels. However, the successful printing zone was limited by the 3D printing windows which prepared for the normal maize starch (NMS)/SA composite gels. SAXS data demonstrated that a smaller mesh size contributed to the higher storage modulus (G'). Principle component analysis (PCA) and correlation analysis showed that the geometric accuracy was closely related to the rheological property of the composite gel. The CFD model explained an unevenness of the velocity distribution in the flow channel due to variations in the diameter of the extruded material, which caused an extrusion expansion effect during the 3D printing process. This study provided a theoretical basis for the standardization and quality control of raw materials for starch-based 3D printing.