In this work, an efficiency calibration function model is presented to calculate the efficiency of segment in segmented gamma scanning (SGS) for 200 L nuclear waste drum. Discrete SGS efficiencies are simulated with MCNP for different densities and gamma ray energies. Parameters of function are determined by using multivariate nonlinear regression method with the efficiencies. The SGS efficiency calibration function is constructed to calculate the efficiency matrix. Aluminum silicate with density of 0.310 g·cm^?3, wood fiber of 0.595 g·cm^?3, point sources ¹³⁷Cs with activity of 3.110×10⁵ Bq and ⁶⁰Co of 1.371×10⁵ Bq are used to construct samples of drum for SGS analysis. Result shows: for the extremely heterogeneous radioisotope distribution of only a point source placed at 8 different positions in the drum, errors of reconstructed activities are ?37.68%～31.52%. Overall, the reconstructed activity is in agreement with the true activity. This method effectively and accurately achieves SGS efficiency calculation and reconstruction of activity.