Projectors and large screens are used widely at present.In most cases,the speaker is confined to the side of the computer because the traditional human-computer interactions are realized with keyboard and mouse.To solve this problem,a laser pen interaction input system is presented.Use CCD camera to image the projection screen at first,then transfer the video to computer via capture card and process it real-timely,calculate the screen coordinates and use Windows API functions to mark the screen contents.In the part of laser pen projection point coordinates extraction,it is put forward the adaptive background updates algorithm.Experimental results show that this system has the advantages of convenient operation,strong practicability and wide adaptability.