Abstract: To set up a non-destructive assay mode for barreled radwaste brought by the process of getting nuclear establishment out of mission, some variables were changed using the Monte- Carlo method to calculate the detecting efficiency when the distributing condition was ideal or partly ideal. The calculation result was analyzed to obtain a method to transform nonideal distributing condition to ideal distributing condition. A non-destructive assay mode for barreled radwaste and a method of getting the detecting efficiency were put forward. The comparison of calculating and experimental results indicates that this method is feasible.