Disclosed is an apparatus for removing a thermal sleeve from a cold leg of a reactor coolant system, which removes an unintentionally separated thermal sleeve without pipe cutting, preventing invasion of impurities into pipes and achieving reliable pipe re-welding. The apparatus includes a sleeve removal tool including a corn head formed at a shaft, a pressure plate below the corn head to maximize hydraulic pressure inside a safety injection pipe, a spring connected to the pressure plate to keep the pressure plate unfolded, and a guide wheel to guide the sleeve removal tool into the safety injection pipe, a horizontal movement carrier including bodies connected to each other by a link, a seating rod for seating of the sleeve removal tool, and moving wheels for movement of the horizontal movement carrier, and a vertical movement carrier including first and second anti-separation bars to prevent separation of the horizontal movement carrier.