Abstract: A changeable mechanism includes a contact support, a connecting rod, a connecting shaft, a cantilever, and a changeable component. The contact support has a moving contact therein, and rotates about a shaft. The connecting rod bottom end is connected to the contact support, the connecting rod top end is connected to a cantilever through a connecting shaft. The changeable component is connected to and drives the cantilever through a main shaft, and drives the contact support to rotate through the connecting shaft and connecting rod. Rotation of the contact support makes the moving contact and static contact separate or in contact to realize opening or closing a circuit breaker. The changeable mechanism may switch between a manual operation mechanism and an electrical operation mechanism of a MCB. The changeable component does not dispose the electrical operation mechanism outside the MCB, so that height and volume of the MCB is reduced.