A new designed FED system was introduced in this article and the components of the system were illustrated. The factors that affected the temporal resolution of the system were analyzed in detail,such as the material and thickness of the photocathode,electric field near the cathode,focusing method,deflection sensitivity and sweeping speed. Finally,designed a FED system with 35 nm-thickness Ag film,1 0 kV / mm electric field near the cathode . Magnetic focusing was used and the deflection sensitivity was 26.3 mm/kV,the sweeping speed could reach up to 1.45×10⁸ m/s,so the total temporal resolution was reduced to about 500 fs.