To address the challenges of interfacial defects and thermal mismatch in heterogeneous fiber splicing for mid-infrared all-fiber lasers, a multiphysics-coupled modeling and parameter co-optimization methodology was proposed. By constructing an asymmetric splicing model, we revealed the coupling mechanisms between thermal gradient distribution, material properties, and fiber dimensions. A parameter optimization methodology for splicing experiments was established through numerical simulations, achieving low-loss (0.15 dB) and high-strength (278 g) splicing of silica/fluoride/fluorotellurite fibers. Experimental results demonstrated that the optimized heterogeneous fiber splice joints achieved high-power transmission across multiple bands (>23.2 W @ 1976 nm, >100 W @ 981 nm) and enabled a fully fiberized 2.8 μm laser system with an output power of 20.3 W. Accelerated aging tests confirmed the system's long-term stability (0.37% power fluctuation @ 10.2 W over 1 hour) and validated that the splice joints met high-power damage resistance thresholds.