Abstract: According to the feature of real-time multi-tasks software systems and nonhomogeneous distributed that different task consume system time,taking task modular as basic testing unit,this paper presents a evaluation model of real-time multi-task software systems.Since the parameters,for example task period,task switch method and switch frequence are easy to analyse and test,the model possesses good extensibility and feasibility.This modeling method provides a new way for the reliability evaluation of real-time multi-task software.