In this paper, we establish the critical version of discrete weighted Hardy-Littlewood-Sobolev inequal-itywithp=q=2, λ=l andα+β=0in R1:∑-N≤r,s≤N;r≠0,s≠0;r≠s(1/|r|α*arbs|s|α/|r-s|≤CαλαN‖a‖2‖b‖2)where a = (a-N,... ,aN) and b = (b-N,... ,bN). When a ≥ 1, we obtain that the sharp estimate λαN is Nα- 1/2.∑-N≤r,s≤N;r≠0,s≠0;r≠s(1/|r|α*arbs|s|α/|r-s|≤CαNα-1/2‖a‖2‖b‖2...