To improve the shortcomings of peak over threshold (POT) extrapolation method and reflect the diversity and dynamics of road conditions, an improved POT extrapolation method to reconstruct the load sequence in the middle of threshold is proposed. Based on the improved extrapolated load, the distribution characteristics of mean and amplitude are analyzed and the two-dimensional joint probability distribution is established. To discuss the influence of low load strengthening characteristics on fatigue life under random load, based on the two-dimensional joint probability distribution, the theory of low load strengthening is introduced in compiling load spectrum. The fatigue life predicted by 8-level load spectrum and 10-level strengthened load spectrum are compared with the test results, respectively. Which verifies the effectiveness and accuracy of life prediction based on improved POT extrapolation and low load strengthening under random load.

为改善峰值超阈值（POT）外推法的不足，反映路况的多样性和动态性，提出了一种改进的POT外推法来重构阈值中间的负载序列。在改进外推载荷的基础上，分析了均值和幅值的分布特征，建立了二维联合概率分布。为探讨随机载荷下低载荷强化特性对疲劳寿命的影响，基于二维联合概率分布，在载荷谱的编制中引入低载荷强化理论。将8级载荷谱和10级强化载荷谱预测的疲劳寿命分别与试验结果进行比较。