To support vehicle crash simulation in lightweight automotive design, this study develops high-precision constitutive and fracture models for a giga casting Mg-7Al-1.5La-1Zn magnesium alloy. Through mechanical testing and microstructural analysis, the plastic hardening, strain-rate strengthening, and ductile fracture behaviors are systematically investigated. A weighted mixed hardening model combining the saturation-type Hockett–Sherby and unsaturated Swift models is established. To overcome the limitations of the classical Johnson–Cook (J-C) model in capturing strain rate–plastic strain coupling, a modified dynamic increase factor incorporating equivalent plastic strain is proposed. Comparative fitting with the Cowper–Symonds model confirms that the modified J-C model better captures the twinning deformation mechanism under high strain rates and achieves higher accuracy. Among three fracture models Johnson–Cook (J-C), Modified Mohr-Coulomb(MMC), and Damage Initiation and Evolution Model (DIEM), the MMC model, which incorporates both stress triaxiality and Lode angle, shows the best predictive performance, with an average accuracy of 86.46% for fracture displacement across the full stress range. Microstructural characterization reveals grain refinement and dispersed Al₁₁La₃ precipitates that improve grain boundary properties. The established parameter set and calibration methods provide a reliable reference for material card calibration in crash simulation of giga casting magnesium alloy body structures.