Abstract: In order to evaluate the reliability index of the longlife products in a short period of time, constantstress, progressive stress and stepstress accelerated life tests are usually used to speed up failure products and estimate their indicators. In view of stepstress accelerated life test, in multiparameter accelerated life equation, aiming at the minimun sumation of every failure mechanism and the variance of the MLE, optimal design model of Type II stepstress accelerated life tests, through the theoretical derivations, the corresponding optimal design plan is obtained. The main conclusion is that for a stepstress test which satisfies the condition of intercept data is monotonic.The optimal plan making minimun of the variance of the MLE is: The intercept data is equal to every stress.The results are the expansion and extension of simple stepstress accelerated life tests, which would guide and organize experiments, improve the accuracy of evaluation findings. It is of theoretical and practical value.

Key words: reliability analysis, stress, optimal design, Type II, failure machanism.