FCC/B2 dual-phase multiprincipal element alloys combine high strength with good ductility, but their insufficient impact toughness under dynamic loading remains a key obstacle to engineering applications. To overcome this limitation, as-cast AlxCr10Fe53-xNi36Mo1 (x=13,15,17 at.%) alloys with different phase volume fractions were designed and fabricated by tuning the Al/Fe ratio, with a size of φ100 mm ×400 mm and a weight of 25 kg, aiming to establish composition-microstructure-property relationships. Microstructural characterization reveals that decreasing the Al/Fe ratio drives a transition from a regular lamellar eutectic structure (Al17) to a hypoeutectic microstructure with primary FCC dendrites (Al13) while simultaneously increasing the FCC phase fraction from 68.1% to 93.1%. In addition, the FCC/B2 interfacial density decreases and gradually deviates from the Kurdjumov-Sachs (K-S) orientation relationship. These structural changes significantly improve the mechanical response: the Al13 alloy achieves the most favourable balance, with ayield strength of 622 MPa, 31.9% tensile ductility, and an impact energy of 19.62 J at room temperature.