Objective To investigate the stress distribution of the knee joint at 0 and 15 days after anterior cruciate ligament reconstruction (ACLR) during 1s and 600s after a compressive force through the axis.of the femoral shaft onto the proximal femur. Method A three-dimensional finite element model of the human knee joint incorporating viscoelastic material properties was developed. The one-dimensional Prony series viscoelastic constitutive model parameters for articular cartilage, menisci, ligaments, and anterior cruciate ligament (ACL) grafts were determined by fitting experimental creep curves. Since no human data currently exist on viscoelastic property alterations in ACL grafts following ACLR, our previous animal study demonstrated that in rabbit models at 15 days post-ACLR, the initial relaxation modulus, equilibrium modulus, and viscosity coefficient of grafts decreased to 60.93%, 60.52%, and 87.85% of the grafts at day 0 after ACLR. Based on these findings, the viscoelastic parameters of human ACL grafts at 15 days post-ACLR were extrapolated. Finite element simulations were then performed to analyze the von Mises equivalent stress distributions in knee ligaments, ACL grafts, articular cartilage, and menisci under a vertical downward compressive load of 1500 N applied to the femur, with loading durations of 1 second and 600 seconds.. Results Following 1s of loading, the peak stress values of femoral cartilage were 0.71MPa(intact), 0.72MPa(0-day after ACLR), 0.69MP(15-day after ACLR). Corresponding tibial cartilage stresses measured 2.71 MPa, 2.77 MPa, and 2.68 MPa, respectively. The medial meniscus demonstrated uniform peak stress of 0.01 MPa across all conditions, while lateral meniscus stresses were 0.05 MPa (intact), 0.04 MPa (0-day after ACLR), and 0.09 MPa (15-day after ACLR). Stress in the ACL or graft revealed 1.05 MPa (native ACL), 1.59 MPa (graft at 0-day after ACLR), and 1.12 MPa (graft at 15-day after ACLR). After 600 seconds of sustained loading, the peak stress values of femoral cartilage were 0.90 MPa (intact), 0.91 MPa (0-day after ACLR), and 0.90 MPa (15-day after ACLR). That of tibial cartilage were 1.46 MPa , 1.45 MPa, and 1.69 MPa, respectively. The peak stress values of medial meniscus were 0.11 MPa across all conditions, with lateral meniscus stresses were 0.14 MPa (intact), 0.13 MPa (0-day after ACLR), and 0.16 MPa (15-day after ACLR). The ACL or graft revealed 0.56 MPa (native ACL), 1.10 MPa (0-day graft), and 0.71 MPa (15-day graft). Conclusion The finite element modeling study incorporating viscoelastic material properties revealed that at 15 days post-ACLR, the initial relaxation modulus and equilibrium modulus of human ACL grafts remained elevated compared to native ACL tissue, resulting in significantly higher stress concentrations within the grafts relative to healthy ACL. However, under both short-term (1 s) and prolonged (600 s) loading durations, despite the compromised mechanical properties of the grafts post-ACLR, the vertical downward compressive force applied to the femur exhibited minimal biomechanical impact on articular cartilage and meniscal structures.