Abstract: At present, in degradation tests, product failure is generally defined as degradation of performance below or above a specified critical value (that is called single point degradation). Although this definition is simple and practical, it is not reasonable enough and degradation failure of the product can not be completely described. In this paper, a single point degradation model is improved, and an interval degeneration model is proposed. We discuss the interval degradation model when the degradation path is liner, and obtain life distribution functions for all kinds of linear interval degradation model. Numerical integration and Monte Carlo simulation methods are used to analyze and compare the life distribution of the interval degradation model and the single point degradation model, and the relationship between the interval degradation and the single point degradation is revealed. Finally, an real data example is analysis to show that interval egradation is more reasonable and effective in practice.