Abstract: The efficiencies and energy spectra of a large volu me welltype NaI(Tl)detector which is used in 4πγ counting method are calculate d by using MonteCarlo method. At the same time, the detector is calibrated by using a series of single γenergy sources. The two results are in good agreeme nt, and the biggest discrepancy does not exceed ±8%. The calculated results are reliable.