An auxiliary lock includes an inside housing mounted to a lock cylinder and a first flange extends axially from one of two ends of the inside housing. A mounting plate is mounted to the lock cylinder and contacts the inside housing. The mounting plate includes a second flange which is mounted to outside of the first flange of the inside housing. The lock cylinder is protected and reinforced in the radial direction by the overlapped first and second flanges. When the inside housing is hit, the inside housing does not expand outward to be pried up. The reinforcement structure is made by way of pressing to use less material and to reduce the manufacturing cost.