With the development of high power microwave technology, the demands for electron beam repetition frequency, current density, response time and emission uniformity are higher and higher. Carbon nanotube (CNt) cathode has been widely investigated, because of its special structure and excellent field emission characteristics. CNt cathode is regarded as a thin film high current cathode, and the interface bonding will affect vacuum performance, stability, lifetime and repeat ability. The direct growth of CNt is a simple and effective means for preparing cathode. When electron energy reaches 1 MeV and the pulse upward gradient attains approximately 60 kV/ns, for CNt cathode, its the emission beam intensity reaches 15 kA and the peak bundle density attains about 1 kA/cm2, the response between beam voltage and current is fast. With the increase of repetition frequency, the emission stability decreases gradually. When the emission power is 15 GW and the emission stability repetitive frequency is 50 Hz, the cathode emission is stable. However with the increase of frequency, the stability becomes weak. When the repetition frequency reaches 100 Hz, voltage and current are almost split into two sections, and the delay time is obviously different. The relation between the voltage and the current meet the exponent law, which is different from the field emission characteristic. After a 1000 shot emission, the morphology of CNt cathode is intact, desorption from the interface of CNt does not happen. So the emission mechanism is flashover plasma emission. Through analyzing the experimental data and considering the plasma expansion effect on diode gap, the plasma speed can be estimated to be about to 3.9 cm/μs.