To reduce the vulnerability of attack helicopters under fragment/projectile strikes, the method of assigning vulnerability index from the whole aircraft to critical components under single and multiple attacks is mainly studied. Under a single strike, the system components in the same direction are divided into three situations: no component overlap, nonredundant model with component overlap and redundant model with component overlap. Two index allocation methods based on the ratio of the presented area of the critical components and the ratio of the vulnerability assessment results of the critical components are proposed. The system components are divided into redundant components and non-redundant components, and an index allocation method based on the proportion of critical components' vulnerability results is proposed under multiple strikes. On this basis, combining with the vulnerability reduction measures of attack helicopters, the vulnerability index requirements of corresponding components are achieved through iterative analysis. Finally, the AH-64D helicopter is subjected to simulation tests under single and multiple strikes, and the vulnerability indexes of B-level and C-level are assigned, which verified the feasibility of the method.