It is very important to select a truncating point in the truncated sequential posterior odd test (SPOT), but there is no efficient method to solve the problem up to now. In the paper, the authors apply the optimization techniques to determine the truncating point. Having avoided selecting C = 1,this method can overcome the difficulties in selecting the stop time N and truncating point C, and make the selection of C more theoretically. The optimal truncating point minimizes the risks undertaken by both sides become and makes them more acceptable.