Based on the principle of field emission, the internal and external factorswhich affected field emission current were discussed in the paper. The high electric field on cathode surface was the necessary condition to generate emission current, but the imprecision of emission current ,which caused by non-uniformity of pixels and non-linearity of field emission properties, led to the difficulty in controlling of luminance. The structure of cathode driving circuit affected control properties of emission current, because itwas a part of emission current loop. The analysis showed that current drivingmode could control anode current exactly.