Diallylbisphenol A (DABA) was synthesized and used as a toughening curing agent for the fabrication of high-temperature bismaleimide (BMI)/carbon fiber composites. The aim of this study was to investigate the effect of DABA content on the curing behavior, thermal stability, mechanical performance, and ablation resistance of BMI/DABA resin systems in order to identify an optimized formulation for high-temperature composite applications. Differential scanning calorimetry (DSC) revealed that the BMI/DABA resin system exhibited a curing onset temperature of 170.1 °C, which is lower than the conventional curing temperature of BMI. Thermogravimetric analysis (TGA) was conducted on three BMI/DABA molar ratios (1/1, 1/0.86, and 1/0.77), together with a phenolic resin for comparison. Among the investigated formulations, the BMI/DABA (1/0.86) system exhibited 5% and 50% weight-loss temperatures (T5% and T50%) of 391 °C and 598 °C, respectively. Oxyacetylene ablation testing showed that the BMI/DABA (1/0.86) composite exhibited a remarkably low weight loss of only 1.9%, indicating excellent ablation resistance. Interlaminar shear strength (ILSS) measurements further demonstrated that the BMI/DABA (1/0.86) composite achieved the highest ILSS values, reaching 61.34 MPa at room temperature and 58.66 MPa at 78°C. These results demonstrate that the optimized BMI/DABA formulation provides an excellent balance of thermal stability, mechanical performance, and ablative resistance, making it a promising matrix resin for high-temperature aerospace and thermal protection applications.