Reliability index assignment is an important process in reliability design. On account of some engineering designers with less experience and irrationality for reliability index assignment, according to the characteristics of small-caliber artillery ammunition, the efficient principle and methods are introduced for adaption of the reliability index assignment in this paper. Taking the modification process in developing an aero demolition and incendiary bomb as an example, to analyze and improve tactical and technical indexes such as reliability and blasting power, the reliability index assignment and calculation were carried out by experts grading method for the reference to application