In　this　paper,　we　study　stability　and　convergence　of　fully　discrete　finite　element　method　on　large　timestep　which　used　Crank-Nicolson　extrapolation　scheme　for　the　nonstationary　Navier-Stokes　equations.　This　approach　bases　on　a　finite　element　approximation　for　the　space　discretization　and　the　Crank-Nicolson　extrapolation　scheme　for　the　time　discretization.　It　reduces　nonlinear　equations　to　linear　equations,　thus　can　greatly　increase　the　computational　efficiency.　We　prove　that　this　method　is　unconditionally　stable　and　unconditionally　convergent.　Moreover,　taking　the　negative　norm　technique,　we　derive　the　L-2,　H-1-unconditionally　optimal　error　estimates　for　the　velocity,　and　the　L-2-unconditionally　optimal　error　estimate　for　the　pressure.　Also,　numerical　simulations　on　unconditional　L-2-stability　and　convergent　rates　of　this　method　are　shown.　(C)　2017　Elsevier　Ltd.　All　rights　reserved.