Disclosed herein is a method for calibrating a radiometric level gauge including a radiation source unit and a detection unit that are disposed across a vessel, the method including: a first step in which different liquid levels of a measurement object in the vessel are measured by the gauge to determine a first relationship between measured values and the liquid levels; a second step in which measurement is performed by the gauge every time when a shielding plate is inserted to change an overall thickness of one or more shielding plates interposed between the radiation source unit and the vessel when it is empty so as to block the radioactive ray in a stepwise manner to determine a second relationship between measured values and the overall thickness of the one or more shielding plates, and then a correlation between the overall thickness of the one or more shielding plates and a liquid level is determined by linking the second relationship with the first relationship; and a third step in which calibration is performed by checking, against the correlation, a simulated liquid level measured by the gauge at a time when one or more shielding plates having a certain overall thickness are interposed between the radiation source unit and the vessel when it is empty.