The kill probability of single air defense missile depends on the process from launching to blasting when the missile hits the target. To analyze the collectivity process of air defense, the collectivity probability of air defense missile was presented in this paper. Two functional models included in the process of air defense operation were improved. In order to joint the four functional models, the concept of radar influential error was found. Then, the collectivity expression of kill probability for missile was extracted. Finally, an example which simulated computation of kill probability for missile was taken.