A strip of step-like base metal was reserved and served as filler metal in dual beam laser fillet welding of T-shape joint of 1.2 mm thick TC4 titanium alloy. First of all, step size was optimized. Then, dual beam laser process parameters were optimized under the optimized step size, based on the results of optical microscope observation, scanning electron microscope observation, tensile test and microhardness test of samples achieved under different laser power, welding speed and defocusing distance. The results showed that when the step size was 0.8 mm×0.8 mm, the surface of welding seam was flat and smooth, and there was no sag or bulge. The increase of laser power or the decrease of welding speed increased weld depth and bonding surface. Satisfactory weld shape with a balance between weld depth and bonding surface area can only be obtained when the welding speed and laser power were reasonably matched. The microhardness test results showed that the microhardness of the heat affected zone and the weld were both higher than that of the base material.When the weld depth is moderate, the axial tensile strength of the T-joint was effectively enhanced, reaching more than 90% that of the base metal. The research results are of great significance to reduce the body weight and promote the integrated technology innovation of aircraft assembly in China.