To monitor the imaging quality of the large-scale collimator, a new method for monitoring the large-scale collimator in real time is proposed, and feasibility of the method is verified. Based on the principle of autocollimating detection, the method uses small plane mirrors to monitor the imaging quality of collimator in real time. The error of the rotating angle of small plane mirror within one focal depth is calculated, and the method demonstrates that the error of the rotating angle of small plane mirror can be controlled within the 0.2″ in one focal depth by using an autocollimator whose accuracy is 0.02″. It plays a role in real time monitoring to the large-scale collimator. The large-scale collimator is widely used for the space remote sensing equipment, and the method for monitoring the collimator will act an active role in the engineering.