Abstract: When to detect an air target in infrared, bird is a potential source of false alarms, its IR characteristics are similar to the target's. The way to calculate the signal of the bird and target on the detector was firstly discussed, and the effects of the IR system's focus position were specially considered. Secondly, the influences that the flight altitude, area and focus position of system have on the bird's signal on the detector were analyzed according to the obtained formula. The signal of the target was also analyzed through the same way. The results show that the bird and the target can not be discriminated by the signal characteristics. Lastly, the way to discriminate the bird and the target was proposed based on the velocity, and the formula of probability that the bird would be treated as a false alarm was deduced and the calculation examples were done. From the results, it can be seen that the bird can be eliminated largely by the differences in velocity.