Quantitative Fault Tree Analysis (FTA) is very effective in analyzing the reliability and safety of space camera software system.Aiming at the difficulty to get information of system faults and construct of fault tree,a method establishing software fault tree based on Rough set is put forward.Because software failure cause is anfractuous and available data are hard to get,a modelling method for bottom event is proposed based on Vague set to calculate the relative significance of bottom event and the occurrence probability of top event.Then,analyzed result from the model is used to guide design of software reliability.Experimental results show that the new method of software FTA can help designer improve system reliability by fault tolerance and error design.The results also show that the new method makes software quantitative analysis possible and optimizes the precision of software measurement.