An improved calibration method that just needs to calibrate one camera parameter was proposed to calibrate a positing and monitoring system for the objects with long distances, larger view field and binocular vision. By considering the leveling device of a vision sensor and combining the condition that the focal length of the camera was fixed in the field，the measurement model was simplified, and parameters to be measured were reduced. The system calibration could be completed when the geographic coordinates of the measuring area center and the cameras, as well as the center coordinates in the image were obtained. With no need for a higher precision target, the effect of target movement and layout on the calibration can be ignored. In application to monitoring an area of 400 m×800 m at 2 km away, the relative error is less than 0.25%, in which that caused by the error of the focal length is less than 0.07%. The proposed method overcomes the difficulty brought by the movement of the target. The improved method could ensure the accuracy, and it is easy to be realized with less time and is suitable for measuring the coordinate in a large field of view.