大多数电子产品可认为是由指数分布单元组成的典型系统，分析其所需备件数量具有工程实用价值，然而如何精确快速地求出备件需求量是目前亟待解决的问题。在假定备件更换时间忽略不计条件下，采用泊松过程方法，推导出系统故障概率的积分表达式，据此提出了系统不能工作时间和平均不可用度的解析分析方法。由于该解析分析公式是级数形式的解析解，因而具有计算速度快、计算精度高的特点，避免了繁琐的蒙特卡罗仿真计算，从而为相同指数分布单元组成的典型系统的备件数目分析提供了工程实用方法。

Most electronic products can be viewed as the typical systems consisted of units with exponential distributionand thus its of great significance to determine the quantity of spare parts.Howeverhow to acquire the needed quantity of spare parts exactly and quickly is a problem in engineering.Suppose that the replacement time is neglectedan integral expression of failure probability is given by using Poisson processbased on which an analytic method is proposed for downtime and average unavailability of the system.Since the formula is expressed as analytical solution in form of seriesit has the characteristics of high calculation speed and high accuracywhich can avoid complicated computation of Monte Carlo simulation.Thereforeit provides a practical method for spare part determination.