In order to reduce influence on early warning results caused by the system error in the early warning system, the calibration to the instrument must be carried on. So it is necessary to discuss the line of sight measuring error farther. This article theoretically discussed the sight error influence on early warning results, including trajectory launch point, trajectory course, and trajectory falling point, and found that the most important influence factor was frequency change. Based on the constant error, linearity error and periodicity error, the corresponding simulation was given to provide the support for the geometry calibration plan.