To assess the reliability of initiating devices accurately with small samples,the influence of up-down test on the scale parameter was studied. The experiment scheme was designed with coefficient of variation. Based on the up-down test data,the revised method of the scale parameter was given. The results of simulation calculation show that the revised parameter has better statistical characteristics and related to the test itself little. The reliability assessment method for initiating devices with small samples based on the bootstrap was proposed,and the estimation of the scale parameter was revised,with that the accuracy of assessment was proved by simulation calculation and test. The method was used to assess the reliability of one initiating device,with γ=0.90 and R≥0.999,the result is that the lower limit reliability is 0.99997,similar to 0.99998 got by the run-down method.