The modern computer and network system, is composed of tens of thousands of components. Any faults in components can affect reliability of the whole system. In the process of running, elements will gradually age and failure rate gradually increase. However, the rate at which these elements age is not synchronous. Thus, in which cases should the elements be upgraded or replaced and in which cases should the elements whose performance is still relatively good be retained, are hard job to handle. In this paper, the author attempts to make an extreme analysis on the reliability of parallel system with N element obeying exponential distribution to improve the system update.