The sources of the prediction error were analyzed; and the distribution and the propagation of the error were considered. The measurement error of the target and the velocity error of the ownship were calculated. The error caused by target maneuvering was considered. Then how to obtain the tracking gate of the minimunm volumn was discussed. The relation between the scanning method of the system and the tracking gate was also analyzed. After the state equation and measurement eqution were formed, a conventional Kalman filter was used for multi target tracking. And the result illustrates that the design of tracking gate is better than before.