In stereo cameras gaze tracking system, cameras can not directly measure the object spatial coordinates outside field of view outside. In order to solve this problem, a system calibration method based on plane mirror is proposed. First, to calibrate stereo cameras to determine the relation between two cameras, a unified world coordinate system is created. Secondly, according to the plane mirror imaging principle, the spatial coordinates of the monitor screen and light source image are decided. The spatial location of light source and screen is determined. The experimental results show that the method has high precision. When the user is away from the screen 500~600 mm, the average accuracy of gazing point can reach 1°, and meet the needs of most gaze tracking applications.