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2 mm thick 5A02 aluminum alloy plate and 2 mm thick TC4 titanium alloy plate were jointed by refill friction stir spot welding (FSpW). The effects of different process parameters on the microstructure and mechanical properties were investigated by changing dwell time and refilling speed. Results show that within the selected process parameters, the sound joints were formed. The fine grains are formed in the stir zone and the grains in the heat thermo-mechanically affected zone are limited to satisfying extent. The atomic diffusion distance at the interface of the sleeve affected zone is larger than the pin affected zone. The atomic diffusion distance and the intermetallic compound (IMC) layer thickness at the interface of the joint are about 1 μm when dwell time is 6s. With the increase of the refilling speed, the tensile shear load of the joint increases. However, the changing trend is not obvious as the dwell time is kept constant at 2s. The tensile shear load strength is the minimum of 4861N at the welding condition of the refilling speed 30mm/min and dwell time 6s. When the refilling speed is 90 mm/min and dwell time is 6s, the tensile shear strength reaches the maximum of 6617N. The shear fracture mode is observed while the macroscopic fracture surface is relatively flat. The microscopic fracture is consisted of dimples.