Laser seal welding of 0.4mm thickness pure titanium with holes was achieved by N d: YAG pulse laser. The effect of laser spot size on the weld geometry of the sealed joint was studied. The results show that full seal welded joint can be achieved when the spot diameter is no more than 0.4mm. The effective thickness of sealing area increases at first then decreases with the increasing of spot diameter. The maximum effective sealing thickness is about 352μm.The sealing joint was consisted of alpha grains with different shapes, and there was a large HAZ in the joint. Besides, with the increase of the spot diameter, the surface melting area of the sealed joint is basically unchanged, but the sealing mode is changed from the keyhole to the heat-conduction type, and the heat-conduction model is more suitable for the hole sealing. Further, calculated laser energy of sealing area shown that there was a positive proportion between the theoretical energy value and laser spot size, but the theoretical analysis does not match with the actual situation. The reason may be that the molten metal beside the hole flow into the hole under the effect of gravity and other forces, which cause the increasing of actual absorption of laser energy.