The shadow mask is the critical component of the colour picture tube and the important component for the choice of colour , its function concentrates on the limitation of electronics bound diameter and the screening direction , the electronics bound which is sent by the electronics gun goes scanning , during the scanning process , we should guarantee every bound gathering into the small holes situated on the screen , then these bounds will point to the regularized position through the small holes on the flat mask , and then three basic colours will be produced , at the same time , those useless electronics will be blocked by the mask board.