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In this paper, a molecular dynamics (MD) simulation model of carbon-fiber/pyrolytic-carbon (C_f/PyC) interphase in carbon/carbon (C/C) composites manufactured by the chemical vapor phase infiltration (CVI) process was established based on microscopic observation results. By using the MD simulation method, the mechanical properties of the C_f/PyC interphase under tangential shear and a normal tensile load were studied, respectively. Meanwhile, the deformation and failure mechanisms of the interphase were investigated with different sizes of the average length <inline-formula> <math display="inline"> <semantics> <mrow> <msub> <mover accent="true"> <mi>L</mi> <mo>&#175;</mo> </mover> <mi>a</mi> </msub> </mrow> </semantics> </math> </inline-formula> of fiber surface sheets. The empirical formula of the interfacial modulus and strength with the change of <inline-formula> <math display="inline"> <semantics> <mrow> <msub> <mover accent="true"> <mi>L</mi> <mo>&#175;</mo> </mover> <mi>a</mi> </msub> </mrow> </semantics> </math> </inline-formula> was obtained as well. The shear properties of the isotropic pyrolysis carbon (IPyC) matrix were also presented by MD simulation. Finally, the mechanical properties obtained by the MD simulation were substituted into the cohesive force model, and a fiber ejection test of the C/C composite was simulated by the finite element analysis (FEA) method. The simulation results were in good agreement with the experimental ones. The MD simulation results show that the shear performance of the C_f/PyC interphase is relatively higher when <inline-formula> <math display="inline"> <semantics> <mrow> <msub> <mover accent="true"> <mi>L</mi> <mo>&#175;</mo> </mover> <mi>a</mi> </msub> </mrow> </semantics> </math> </inline-formula> is small due to the effects of non-in-plane shear, the barrier between crystals, and long sheet folding. On the other hand, the size of <inline-formula> <math display="inline"> <semantics> <mrow> <msub> <mover accent="true"> <mi>L</mi> <mo>&#175;</mo> </mover> <mi>a</mi> </msub> </mrow> </semantics> </math> </inline-formula> has no obvious influence on the interfacial normal tensile mechanical properties.