•Dual laser-beam bilateral synchronous welding (DLBSW) experiments of Ti6Al4V double lap-fillet joints have been carried out.•The relationship of thermal cycle and microstructural evolution is studied.•Narrow HAZ width and fine acicular α'-martensite for low welding speed is observed.•Residual stress distribution has a great influence on fracture behavior of lap-fillet joint.•Slower welding speed is in favor of higher tensile strength. Employing prefabricated “boss” on the substrate to replace wire feeding, an attempt is made to explore the non-filler welding process of Ti6Al4V double lap-fillet welded joints with the technology of dual laser-beam bilateral synchronous welding (DLBSW). The weld quality is evaluated in terms of bead geometry, microstructural morphology and mechanical property. An optimal range of welding speed (1.3 m/min) with constant unilateral laser power of 950 W is identified to achieve satisfactory weldments with no underfill defect and acceptable porosity. Phase transformation and microstructural evolution occurring through thermal history in different regions of the welded joint are clarified. The average length of α'-martensite in the weld seam (WS) decreases significantly as welding speed increases. A connection between dimensional variation of α'-martensite and tensile strength is developed, and the fracture mechanism of the lap-fillet joint is discussed. The existence of fine α'-martensites helps to attain high-performance joints with strength comparable to the base metal (BM). The fracture position of tensile specimens is located in the region near the weld toe where stress concentration occurs. Fractography analysis indicates that the failure of weldments is in ductile mode with the feature of a great number of dimples.