In a field emission display panel, the triode structure has been used to decrease the driving voltage. Because a very strong electric field has been appliedin front of the field emitters, the electron beam usuallydivergences in a normal triode structure[2;3]. A new triode structure with an additional focus electrode has beenproposed. A numerical program has been used to calculate the focus characteristic of this triode structure. Wealso analyze the variation of the current density distribution on the screen with different focus voltages.