Abstract: A theftproof lock, comprising a movable loop 9 mounted on the turning member of the bicycle, a fixed loop 3 mounted on the frame 1, a sheath 11 mounted on the said fixed loop 3, lock rod 12 situated inside the said sheath 11. Said movable loop 9 provides several slots 10, lock tongue inserted in one said slot 10. Said fixed loop 3 mounted on the frame 1 below the upper steel bowl 4, and the upper steel bowl 4 suppressed over the upper section of the fixed loop 3. When the fixed loop and the movable loop are locked together, none of the lock, the fixed loop, and the movable loop can be removed. User can install the lock after buying the bicycle; the lock is suitable to all kinds of bicycle.