Abstract: The non-penetration butt friction stir welding of 10 mm thick A356-T6 plate was carried out under continuous change of butt gap. The tolerance of A356-T6 plate friction stir welding to the gap was studied, and the microstructure, mechanical properties and fracture morphology of the joints were analyzed. The results show that when the gap is less than 0.6 mm, a joint with good mechanical properties can be obtained. When the gap is within 0.3 mm, an ideal joint with no defects inside the joint can be obtained, the welding coefficient is higher than 0.7, when the gap increases to 0.6 mm, the mechanical properties of the joint are slightly reduced, the welding coefficient is 0.65, a small hole defect with size of 0.069 mm appears in the joint fracture, and the fracture under the three gaps shows typical ductile fracture characteristics. When the gap is 0.9 and 1.2 mm, the mechanical properties of the joints are significantly reduced, the fracture has tunnel holes with sizes of 0.134 mm and 0.195 mm respectively, which shows mixed fracture characteristics of ductile fracture and quasi-cleavage fracture.