Explosive　emission　cathodes　(EECs)　are　adopted　in　relativistic　backward　wave　oscillators　(RBWOs)　to　generate　intense　relativistic　electron　beam.　The　emission　uniformity　of　the　EEC　can　render　saturation　of　the　power　generation　unstable　and　the　output　mode　impure.　However,　the　direct　measurement　of　the　plasma　parameters　on　the　cathode　surface　is　quite　difficult　and　there　are　very　few　related　numerical　study　reports　about　this　issue.　In　this　paper,　a　self-developed　three-dimensional　conformal　fully　electromagnetic　particle　in　cell　code　is　used　to　study　the　effect　of　emission　uniformity　on　the　X-band　RBWO;　the　electron　explosive　emission　model　and　the　field　emission　model　are　both　implemented　in　the　same　cathode　surface,　and　the　local　field　enhancement　factor　is　also　considered　in　the　field　emission　model.　The　RBWO　with　a　random　nonuniform　EEC　is　thoroughly　studied　using　this　code;　the　simulation　results　reveal　that　when　the　area　ratio　of　cathode　surface　for　electron　explosive　emission　is　80%,　the　output　power　is　unstable　and　the　output　mode　is　impure.　When　the　annular　EEC　does　not　emit　electron　in　the　angle　range　of　30　degrees,　the　RBWO　can　also　operate　normally.　(C)　2016　Author(s).