Abstract: In order to consider the upper limit of firing and lower limit of no-firing of initiating devices in their reliability design, a method based on the reliability index transformation is put forward. According to the settled risk value of producer and consumer, the reliability index is transformed to the design index of a product. Then the initiating point of 50% is determined with sensitivity distribution model of mechanical initiating devices. Compared with the actual product, it showed that the initiating point can guarantee the reasonable margin of the upper limit of functioning reliability and lower limit of no-firing reliability. Use of this method in a stab detonators reliability design indicated that the method is practicable.