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In the narrow-gap multi-layer welding of thick Ti-6Al-4V titanium alloy sheets, reducing the number of filling layers can effectively improve the welding efficiency and reduce the possibility of interlayer defects. In order to explore the changes in the microstructure and properties of the weld after reducing the number of filling layers, Ti-6Al-4V titanium alloy sheets with a thickness of 20 mm were successfully welded using the oscillating laser beam mode by laser narrow-gap multi-pass wire filler welding in eight, six, four, and three layers, and all of the formations were good. To reduce the number of filling layers and increase the welding line energy from 0.4 kJ/mm to 1.2 kJ/mm, the melting depth and width of the single layer were changed from 4.3 mm to 10.6 mm, and 5.7 mm to 10.3 mm. The average grain size of the needle-shaped martensite increased from 1.83 μm to 2.38 μm, while the tensile strength of the filled weld area decreased from 1301.8 MPa to 1169.8 MPa, which was higher than that of the base metal of 902.1 MPa. Since there are more columnar crystals in the center of the weld at low heat input, the impact energy was 20.53 J (60.6% of the base metal) at room temperature and 15.76 J (65.9% of the base metal) at −50 ∘C. Considering the weld formation, microstructure and mechanical property, welding four layers of fillers obtained with moderate line energy (0.8 kJ/mm) was more suitable.