This paper studies the issues about the X-band super-radiance from a relativistic backward wave oscillator (RBWO) with the central frequency of 9.25 GHz, and the output port of the RBWO is directly connected to a vlasov antenna. The particle simulation code UNIPIC and the self-developed antenna simulation code are combined to simulate the full process of the generation and the radiation of the microwave pulse. Effects of natural gas at difference pressures and injected voltage pulse on the working characteristics of RBWO are simulated and discussed. Simulated results indicate that the peak value of the output power can achieve 3.68 GW, and the instantaneous efficiency can exceed 100%. When the tilted angle of vlasov antenna is 20 degrees, the gain of the antenna is 15.5 dB. The power density can reach 0.728 W/cm2 at the far distance of 1 km.