Abstract: The system of synthetic identification friend or foe contains a lot of detection means, such as radar system, infrared sensors system, visible light sensors system, exterior information supporting, and so on. Evaluating the system of synthetic identification friend or foe, analyzing the influencing factors of the system of synthetic identification friend or foe and giving the quantitative conclusion is a complicated and difficult problem. Radar is one of the important detecting equipments in the system of synthetic identification friend or foe. Its basic functions can be attributed to target searching, target locating, target recognition, information transmitting, and so on. Its performance is determined by availability, surviving ability, anti jamming ability, detecting ability, target searching ability, target locating ability, data processing ability, target recognition ability, and a lot of other influencing factors. To evaluate radar system in the system of synthetic identification friend or foe reasonably, the key problem is considering the influencing factors mentioned above and modeling with these factors. The model should reflect the different influences caused by different factors and should reflect the different influence degree caused by them. The availability is one of the most important influence factors in radar system. If the availability of the radar system is poor, none of the other radar functions can act properly, even if they are excellent. So it is very important to evaluating the availability of radar system. The paper builds mathematical model only for the availability of radar system. Supposing that the times of the malfunction of the radar system is a poisson process, the paper puts forward a worked example and an evaluation method. Simulation results are given to demonstrate the validity of the modeling method. It provides a useful reference for the general evaluation of radar system under the condition of synthetic identification friend or foe.