Abstract: CO₂ gas shielded welding was carried out on Q690D high-strength steel plates with a thickness of 4 mm by using ER50-6 welding wire. The influence of welding gap （0.75, 1.00, 1.25, 1.50 mm） on the microstructure and properties of the welded joints was studied. The results show that with the increase of the welding gap, the residual height and melt width on back side of the weld increased. When the weld gaps were 1.00, 1.25 mm, the weld formation quality was good. When the weld gap was 0.75 mm, the defect of incomplete penetration appeared on the back side of the weld. When the welding gap was 1.50 mm, defects such as weld bead and weld slag appeared. Under different welding gaps, the joints were composed of the weld, the heat-affected zone and the base metal zone, and the heat-affected zone was divided into the coarse-grained zone, the fine-grained zone and the incomplete recrystallization zone. With the increase of the welding gap, the dynamic recrystallization degree of the weld first decreased and then increased, the average geometric dislocation density first increased and then decreased, and the proportion of high-angle grains first increased, then decreased, and then increased. The dynamic recrystallization degree was the highest, and the geometric dislocation density was the lowest under a welding gap of 1.50 mm; the proportion of high-angle grains was the smallest under a welding gap of 1.25 mm. The hardness of the fine-grained zone of heat-affected zone was the highest, and the hardness of the weld was the lowest under different welding gaps. With the increase of the welding gap, the hardness of both the fine-grained zone of heat-affected zone and the weld first increased and then decreased, the residual tensile stress of the joint increased, and both the tensile strength and the percentage elongation after fracture first increased and then decreased. The optimal welding gap was 1.25 mm. At this time, the hardness of the fine-grained zone of heat-affected zone of the joint and the weld was the highest. The tensile properties were the best; the percentage elongation after fracture was 10.37%, and the tensile strength was 697 MPa, reaching more than 90% of the base metal. The tensile fracture was composed of a large number of cavities and showed the characteristics of ductile fracture.