Abstract: In nuclear waste drum inspection by Segmented Gramma Scanning(SGS), the detector counts are different with the point sources at different positions during the rotation process, and the attenuation distance needs to be calibrated. We used the Matlabs data fitting method and numerical calculation method to get the corrected attenuation distance. The Monte-Carlo code was used to simulate the radionuclide in a waste barrel uniformly filled with polyethylene material with density of 1.0 g/cm3. Then the count distributions of 137Cs point sources at seven different locations in the barrel were obtained. The detector counts were simulated every 30 degrees on seven different rotational circles, and the corrected average attenuation distance was given by calculating the partial area with high contribution rate. The relative error of the activity estimation obtained by this method is between 0.6% and 25.2%. Compared to traditional methods (the relative error of the activity estimation is between 11.8% and 209.9%), the error is decreased obviously.