Abstract: The major factor that leads to the damage of electron beam collector in relativistic backward wave oscillators with high magnetic field is analyzed. By tapering stainless steel collectors, the ability of collectors to bear the impact of high current electron beams is improved. In single shot experiment, the effects of electron beam energy density on the surface damage of stainless steel collectors and the generation of microwave are studied. Based on the experimental results, damage threshold range of stainless steel collectors with respect to electron beam energy density is brought forward with the research on the space density distribution of intense electron beams in the foilless diode.