This　paper　focuses　on　the　stability　and　convergence　analysis　of　the　first-order　Euler　implicit/explicit　scheme　based　on　mixed　finite　element　approximation　for　three-dimensional　(3D)　time-dependent　MHD　equations.　Firstly,　for　initial　data　u(0),　B-0　is　an　element　of　H-alpha　with　alpha　=　1,　2,　the　regularity　results　of　the　continuous　solution　(u,　p,　B)　and　the　spatial　semi-discretization　solution　(u(h),　p(h),　B-h)　are　obtained,　and　L-2-error　estimate　of　(u(h),　B-h)　is　deduced　by　using　the　negative　norm　technique.　Next,　through　the　use　of　mathematic　induction,　the　H-2-stability　of　the　fully　discrete　first-order　scheme　is　proved　under　the　stability　condition　depending　on　the　smoothness　of　initial　data.　Here,　the　stability　condition　is　Delta　t　＜=　C-0　for　alpha　=　2　and　Delta　th(-1/2)　＜=　C-0　for　alpha　=　1where　C-0　is　some　positive　constant.　Then,　under　the　stability　condition,　the　optimal　H-1-L-2　error　estimate　of　the　fully　discrete　solution　(u(h)(n),　B-h(n))　and　optimal　L-2-error　estimate　of　the　fully　discrete　solution　p(h)(n)　are　established　by　using　the　parabolic　dual　argument.