This　paper　considers　a　Crank–Nicolson　extrapolation　scheme　based　on　mixed　finite　element　method　to　solve　the　three-dimensional　incompressible　magnetohydrodynamics　(MHD)　equations.　We　prove　that　the　fully　discrete　scheme　is　almost　unconditionally　stable　and　convergent,　i.e.,　stable　and　convergent　when　the　time　step　is　less　than　or　equal　to　a　constant.　By　a　new　negative　norm　technique,　the　optimal　error　estimates　in　L2-norm　are　derived.　Meanwhile,　the　numerical　investigations　provide　a　sufficient　support　for　the　theoretical　analysis.　©　2018　Elsevier　Ltd