The mechanical behavior and fracture failure characteristics of Mg-9Gd-4Y-2Zn-0.5Zr alloy at various strain rates were investigated, including parameter calibration and verification based on the Johnson-Cook (J-C) constitutive model and failure model. Quasi-static tensile tests at different temperatures were conducted by a universal testing machine, while dynamic tensile tests at high strain rates (1000–3000 s^-1) were performed by a Hopkinson bar apparatus. Based on the experimental data, modifications were made to the strain rate hardening and thermal softening terms of the J-C constitutive model were modificated, and relevant model parameters were calibrated. Further numerical simulations were carried out; the fracture locations and true stress-strain curves between experimental and simulated results were compared to validate the reliability of the failure model parameters. The fracture morphology of the magnesium alloy was observed and the microstructural characteristics influencing failure under different temperatures and strain rates were explored. Both dimples and cleavage steps were observed in the fracture morphologies during quasi-static and dynamic tensile processes, indicating a mixed fracture mechanism. Slightly more cleavage steps are found at higher strain rates, which is related to the strain rate sensitivity of the magnesium alloy. In contrast, ductile fracture is predominant during high-temperature tensile tests.