A method for forming a protective structure of active matrix triode field emission device is provided. The method comprises the steps of forming a silicon active region; depositing a gate oxide layer over the silicon active region; depositing and patterning a first metal layer over the gate oxide layer; doping impurities into portions of the said silicon active region to form a source/drain in a first conductive type and simultaneously to form a diode having a terminal in the first conductive type; forming ILD layer over the first metal layer and forming a plurality of contact holes thereon; depositing and patterning a second metal layer; forming a passivation layer over the second metal layer and forming a plurality of via holes thereon, depositing and patterning a third metal layer to form a gate and a tip structure.