Key structure and mode switching method thereof

A key structure includes a circuit board, a membrane, a positioning frame, a sensor module, a scissor leg, and a key cap. The membrane is disposed on the circuit board and is electrically connected to each other. The positioning frame is disposed on the membrane. The sensor module is disposed on the membrane and is electrically connected to the circuit board. The scissor leg is connected to the positioning frame and is adapted to move up and down relative to the positioning frame. The key cap is detachably disposed at the scissor leg and is spaced with positioning frame. The key cap has a reflective plane and a shaft component. The reflective plane faces the membrane. The shaft component is extended from the reflective plane and penetrates through the scissor leg and the positioning frame to abut the membrane. The sensor module is aligned with the reflective plane.