Abstract: A three-axis electro-optical tracking system can be used to track any space target in all azimuth. It can rapidly and accurately track the target in the altazimuth zenith blind region by rotating X axis. Several determinant methods to track the targets in the alt-azimuth zenith blind region by the three-axis electro-optical tracking system were introduced, such as target range determinant method, Z axis angular speed determinant method, preset blind region determinant method and so on. Determinant methods to track targets in the blind region at zenith have a directly effect on the tracking accuracy of the electro-optical tracking system with three-axis gimbal. Analysis and simulation results prove that the determination of the entry and exit points in the alt-azimuth zenith blind region with maximum angular velocity of Z axis could help three axes trackers to achieve better tracking accuracy and smooth transfer of tracking mode.