Abstract: This paper introduces the studies on the relativistic multi-beam electron gun with different structures. The theoretic analysis and experimental results show that the spot distortion is obvious with fan-shaped multi-beam cathode due to the nonuniformity of the surface electric field. Furthermore, the fan-shaped beam rotation around the beam center, caused by the effect of space electromagnetic field, exasperates the beam distortion and reduces the transmission efficiency evidently. Whereas, the electric field on the surface of cylindrical multi-beam cathode is more uniform, which restrains the spot distortion. Meanwhile, the self-center rotation can not change the beam profile with the columned form. Considering the emission from the multi-cathode rods and cathode base, the transmission efficiency of the multi-beam is limited. In addition, the strong beam impaction on the multi-beam waveguide could lead to intense current transport cutoff. It is effective to increase the transmission efficiency by enlarging the inner radius of magnetic system. At present, the output power of multi-beam klystron can get to the order of gigawatt with multi-beam power 6.5 GW and transmission efficiency 89%.