In this paper, we investigate the following fractional Kirchhoff system in R N : { ( 1 + b ∫ R N | ( − Δ ) s 2 u | 2 d x ) ( − Δ ) s u + V 1 ( x ) u + λ 1 u = μ 1 | u | p 1 − 2 u + r 1 β | u | r 1 − 2 u | v | r 2 , ( 1 + b ∫ R N | ( − Δ ) s 2 v | 2 d x ) ( − Δ ) s v + V 2 ( x ) v + λ 2 v = μ 2 | v | p 2 − 2 v + r 2 β | u | r 1 | v | r 2 − 2 v , under the constraints ∫ R N | u | 2 d x = a 1 2 , ∫ R N | v | 2 d x = a 2 2 , where 0<s<1, 2s<N<4s, b, μ 1 , μ 2 , β are all positive numbers and r 1 , r 2 ≥ 1 . V 1 ( x ) , V 2 ( x ) ≥ 0 are the potential functions and a 1 , 0 $ ]]> a 2 > 0 are prescribed constants. In L 2 -critical case, we first obtain the existence of non-negative solutions through a constrained minimization problem. Then under some further assumptions on r i and V i ( x ) for i = 1, 2, we conclude the solution is Hölder continuous and decays like | x | − ( N + 2 s ) at infinity. Finally, the positivity of solutions is verified. In L 2 -supercritical case, the normalized solutions are obtained by a minimax theorem. Precisely, the theorem allows us to obtain a bounded Palais-Smale sequence satisfying the Pohozaev identity in limit sense. Further, the signs of Lagrange multipliers are determined to ensure the strong convergence of the approximate critical point sequence in H s × H s .
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