The Al-AlN granular film was proposed as a cathode emitter of surface-conduction electron-emission display (SED) and the effect of Al-AlN granular films’ resistivity on electroforming in experiment and simulation methods was studied. Electroforming could be successfully completed with appropriate Al-AlN granular film resistivity between 1.98 and 15.10 mΩ$cm, and the corresponding turn-on voltage of electroforming increased from 6.2 to 10.5 V with the resistivity increasing. In addition, a temperature profile on Al-AlN emitter was simulated and the temperature decreased from middle to two sides, which were corresponding to surface morphology of Al-AlN emitter after electroforming.