Surface conduction electron emission device (SED) was fabricated with a back gate electrode. And then the device was electroformed in using continuous triangle wave at vacuum environment. After that we prepare an aluminum back electrode on the back of device. With the back electrode voltage changing, we could observe the change of the emission current. When device voltage and anode are fixed, the device’s emission current and efficiency decrease under positive back voltage of 5V. On the contrary, the device’s emission current and efficiency increase with negative back voltage of -5V. Our results directly proved that the emission electrons were from the field include cool electron emission, but not from the power induced thermal electron emission due to size-effect.