A volume calibration phantom comprises a container; a plurality of plates stacking up inside the container; and at least one slab of radioactive source, each of which is disposed between the adjacent plates and comprises a plurality of radionuclides. With the volume calibration phantoms, the present invention further provides a calibration method which is an improvement over conventional calibration methods of space geometric center point source and relative penetration factor ratios. The method comprises the steps of generating a calibration curve of density vs. counting efficiency corresponding to the several different volume calibration phantoms; calculating the density of a radioactive waste specimen to obtain a corresponding radioactive activity according to the calibration curve, and then revising the corresponding radioactive activity according to the energy dependency and equation of gamma gross radioactivity for multiple radionuclides so as to obtain the correct gamma gross radioactivity of the radioactive waste specimen.