Structural materials with excellent toughness, a wide processing window, outstanding mechanical performance, and high thermal stability are highly desired in engineering. This work reports a novel bismaleimide (BMI) resin system fabricated using bis[4-(4-maleimidephen-oxy)phenyl)]propane (BMPP), 1-(2-methyl-5-(2,5-dioxo-2H-pyrrol-1(5 H)-yl) phenyl)-1H-pyrrole-2,5-dione (BTM), and diallyl bisphenol A (DABPA) by a melt method. The behaviors of the BTM/BMPP/DABPA resin were modified by adding vinyl-terminated butadiene acrylonitrile (VTBN) in various amounts. The cured BTM/BMPP/DABPA/VTBN resin system exhibited all of the abovementioned desirable properties. Excellent performance was achieved by the post-cured BMI resin containing 6 phr of VTBN (VTBN-6). The glass transition temperature (Tg) and the 5% weight loss temperature of VTBN-6 were 278°C and 408°C, respectively. Relative to VTBN-0 (BMI resin without VTBN), the impact strength of cured VTBN-6 (12.32 KJ/m2) improved by 45.6%, and the fracture toughness values, KIC and GIC, increased by 48.7% and 26%, respectively. Moreover, the prepolymer of VTBN-6 exhibited low viscosity over a wide temperature range (70–200°C) under dynamic conditions and for an extended time (70 min; 75% improvement over VTBN-0) in an isothermal test. These results confirm the wide processing window of VTBN-6. The high toughness of the VTBN-containing BMI resin was compatible with other excellent performances of the modified resin.