Abstract: The transmission characteristics of ultra-short electron beams of the ultra-fast electron guns in scanning state were analysed, and the deflection distance of the ultra-short electron beams passing the deflection-scan system was calculated. The results indicate that the initial voltage must be within the range of 700-1400 V to counterbalance the effect of the negative scan slope voltage, ensuring that the electron beams will pass the deflection-scan system and finally hit the phosphor screen of 30 mm in diameter, and the cooperative function of the initial voltage and the negative scan slope voltage is one of the two premises of the scan experiment. The other premise is the synchronization of the electrical and the optical signals. Following the discussion of the great difficulty caused by the first promise, an experimental system that can adjust the optical delay more effectively in the scan experiment was designed, which can facilitate the synchronization of the negative scan slope voltage and the ultra-fast electron pulse.