Abstract: Many obscure expressions and uncertainties exit during the process of safety analysis for complicated safetycritical software,while the theory of fuzzy sets and subjective evaluation is an effective methodology to deal with these　problems. We presented the fuzzy expressions of the software safety factors, then analyzed the safety score of single　component. After that, we synthesised the safety score of subsystem and system quantitatively by using the fuzzy operalions and evidential reasoning approach. Finally, an example was presented to demonstrate the proposed software analysis and synthesis method.

Key words: Component based software, Fuzzy sets, Subjective method, Safety score, Synthesis