In order to find out the weakness of the ejection mechanism of launch device, a fault tree for the ejection mechanism of launch device unable to eject was built in this text. A variety of reasons or combination reasons for the fault tree was found out through the fault tree analysis method, the quantitative analysis of fault tree was developed on the basis of the statistical analysis of fault probability of electronic components, structural parts and hydraulic component. The results show that in the bottom events, the important degree of structure and hydraulic components are lower, and the emphasis should be focused on the electronic components in the development.