The present discloses an automatically locked retractable protective door sill, including a retractable curtain assembly, a scroll assembly, an upper fixing base and a lower fixing base, a dial mechanism and a locking mechanism are arranged in the upper fixing base, the locking mechanism includes a hydraulic rod capable of retracting at a fixed time, and the hydraulic rod is connected to the dial mechanism; the sliding base is connected to the hydraulic rod, and the hydraulic rod drives the sliding base and drives the linkage base to reset automatically, so the automatic locking of the locking mechanism is achieved, and then the scroll assembly is locked. The whole device has a simpler structure and more convenient operation, the dial button may be manually pushed to return for locking after not reaching the set time, so as to prevent children opening the door sill during an automatic locking process.