In this study, the Cu-Hf alloy and low carbon steel are respectively welded using a high-power fiber laser. The morphological characteristics of keyholes obtained by freezing and preserving the keyholes in a molten pool are compared. The results show that the keyholes can be retained in both materials, and the diameter of a keyhole is obviously greater than that of a spot. The shape of the keyhole retained in the Cu-Hf alloy is similar to that of a “gourd” with the intersection of big and small rings. The diameter of the small ring located in front of the welding direction is equivalent to that of the spot, and the diameter of the large ring is in the order of a millimeter. In low carbon steel, the laser output time to retain a keyhole is very short, the solidation time of the molten pool is long, and only the large circle area of the keyhole can be retained. Further analysis shows that a keyhole morphology can be divided into two parts: the laser direct-action area and vapor pressure maintaining area. In numerical simulations, the morphological characteristics of a keyhole must be considered when building a suitable heat source model for laser welding.