In order to meet the demand of software maintainability assessment, three-state probability description method was put forward. Hidden Markov Chain model was introduced to estimate maintainability of three-state software, and state transform model was built up. Historical maintenance time of software module was collected from configuration management database to ascertain frequency of success maintenance so as to estimate software maintainability. Affecting factors set value was computed through Fuzzy inference theory. The estimation model was trained and validated through real software case, and the result shows that this method is feasible and repeatable, and can be further studied.