A multistage lock has a stationary frame, an inner frame, an outer frame mounted between the stationary and inner frames, multiple stops mounted on the stationary and inner frames, two sliding panels mounted on the outer frame and a handle assembly connected to the sliding panels. Stopping protrusions of the stops selectively engage limiting protrusions on the sliding panels when the at least one handle is turned. Consequently, the inner and outer frames are selectively opened. The multistage lock has a simplified structure and allows people to decide whether a door panel (window) is open according to situations and weather outdoors.