The poor toughness of bismaleimide (BMI) resin hampers its application in the aeronautics and space field. Herein, a flexible monohydric aliphatic amine (MAA) was introduced to in,situ toughen BMI resins, in which a novel self-assembled second-phase structure was formed. The micro-nanostructure can be adjusted by changing MAA to perfectly improve the comprehensive performance of BMI. In addition, only a small amount of MAA can achieve substantial increase of toughness of BMI. The morphology of MAA-modified BMI was observed via scanning electron microscopy and atomic force microscopy. Consequently, after adding 1% MAA, the elongation at break of MAA-modified BMI increased by 32.3%. Compared to the neat BMI, the flexural strength of MAA-modified BMI exhibited enhancement by 30.1%. Significantly, the presence of MAA in the BMI resin resulted in a remarkable increase in K-IC of 24.3% and G(IC) of 54.2% without sacrificing the processability and heat resistance.