The distribution characteristic of the target spatial state changing with the time is defined as the time spectrum of the target. By using the time spectrum information of the target, including its trajectory, speed, maneuverability, and the space coordinate information, the accuracy of ship to air target threat assessment can be improved. In this paper, the objective approach is used together with the subjective one to judge the state of the aerial targets and achieve the purpose of threat sequencing. Examples show that, the proposed method for warship air defense threat assessment can make reasonable use of reconnaissance information, and provide an effective assistant means to the naval commanders for making decision timely.