Abstract: The Weibull distribution with two parameters is widely used in Aeronautical and Astronauticd products. For complex system with small samples, the shape parameter of Weibull is neither given by a few disable data nor estimated by impersonal method. At present, slove the problem by experts enacting or non-parameter statistic, which is subject to considerable uncertainty. This paper proposed an object and conservative method to estimate the shape parameter, at the same time get the reliability growth model for the system which is developed grading. At last, an example is given to illustrate the usage of the proposed method in engineering application.