An ion source head structure of a semiconductor ion implanter including a filament, a filament clamp, a cathode, a cathode clamp, an insulation assembly is provided. The filament clamp clamps the filament. The cathode presents a shell shape and has a receiving space. The filament is located in the receiving space. The cathode clamp clamps the cathode. The insulation assembly is disposed between the filament clamp and the cathode clamp such that the filament clamp is insulated from the cathode clamp. The insulation assembly has a first surface, a second surface opposite to the first surface, and an outer surface between the first surface and the second surface, wherein the first surface of the insulation assembly is in contact with the filament clamp, and the second surface of the insulation assembly is in contact with the cathode clamp. A step difference exists on the outer surface of the insulation assembly.