Quantitative fault tree analysis (FTA) is very effective in analyzing the reliability and safety of space camera software system. It is difficult to get knowledge about system’s faults and construct the structure of fault tree. A method of software fault tree establishing based on Rough set is put forward. Because software failure cause is anfractuous and available data are got hard

an approach to model for bottom event based on vague set is proposed. The relative probabilistic importance of bottom event and the occurrence probabilistic of top event can be calculated using the model. The outcome is very useful to guide design of software reliability and decide the emphases of software test. Experimental results show that the new approach of software fault tree analysis can help designer advance system reliability develop by fault tolerance and error design. The results also show that the new method makes software quantitative analysis possible which is unable to deal with as a result of reliability data getting difficult.