To meet the demand of key position air-defense operational requirements in the multiple island-reefs, and com.bining the new characteristics of modern air defense of anti-missile operation, as well as synthetically considering themain factors of air striking targets and the relative value of key position, in this paper, the rough set theory was used to ob.tain the minimal invariant set and solve the minimum rule set from much inaccurate, inconsistent, and incomplete informa.tion, derive the index weight, as well as rank the threat level of the targets in the case of being attacked by several targets.So as to reveal the implicit rules in incomplete information, the rough set theory could greatly improve the efficiency of ex.tracting useful information. It was shown that this method could objectively rank the threat level of the air raid targets,which had the certain referent value for the study of the essential air defense operation decision in the multiple island-reefs after the cases analyses.