System safety index in aircraft level is analyzed to solve the problem of index selection and quantization in this paper. The flaws of the safety parameter application in system safety analysis are indicated through the analysis and comparison of domestic safety parameters. Based on characteristic of system safety, the requirment and dimensionality of index are proposed. Using the experience on system safety assessment of foreign countries for reference, and considering reliability, methods to select and quantify safety index in aircraft level is started, and one example is given to illustrate when and how to apply safety index. Research shows, combining safety and reliability, selecting indexes having effect on aircraft safety to conduct system safety assessment in aircraft level, can be a practical and reasonable way to solve problems of traditional safety parameters such as the difficulty of quantification and inapplicability.