Abstract: A small scale internal blast experiment is one of the important methods to investigate an internal blast load. The published small scale internal blast experimental results offered us lots of data but show a wide scatter. In this paper, three dimensional finite element internal blast flow field calculating models taking an explosive location departure into account were established, and were validated with experimental data. Then the influence of explosive location departures on the results of small scale internal blast experiments was analyzed. The results show that the reflected peak overpressure is sensitive to an explosive location departure, and empirical formulas evaluating the peak overpressure variation are fitted. The impulse of internal blast has a slight difference while the explosive location departure near the center of a blast vessel, and the max relative error is less than 13%.