TC4 titanium alloy with a thickness of 8mm was welded by fiber laser operating in beam-weaving mode. The plasma spectrum as well as image analysis of plasma and keyhole during the welding process were acquired by spectrometer and high-speed camera respectively. The reason for suppressing keyhole-type porosity in the weld was analyzed. The result showed that the weaving beam had a remarkable effect on the suppression of keyhole porosity in titanium alloy welding. Under the process condition studied in this paper, the porosity presented in the non-weaving beam welded joint was up to 9.8%, but the porosity in the weaving beam weld was decreased to less than 1%. At the parameter of 5 kW of laser input, 2 m/min of scanning speed, 80 Hz of oscillation frequency and 0.5 mm of displacement, the keyhole-type porosity can be completely eliminated. These evidences including the keyhole image, the spectrum intensity and observations of plasma indicated that the weaving beam makes the keyhole more stable compared to the welding with non-weaving beam. It is suggested that the weaving beam may increase the contact area between the laser beam and the molten pool. Therefore, both axial force and radial force making the keyhole open can be increased.