The present invention relates to a thermowell blocking apparatus for a high-pressure reactor used in a desulfurization process, which can rapidly prevent noxious high-temperature and high-pressure gases from leaking out of a high-pressure reactor when the noxious high-temperature and high-pressure gases leak from the high-pressure reactor due to unexpected damage occurring to a thermowell used for measuring a reaction temperature of a catalyst. The thermowell blocking apparatus can prevent the economic losses caused by the stoppage of the desulfurization process, and can reduce consequential damages to human life, the environment, and the like.