In addition to electric fields and currents, light can also provide an approach to modulate the ferromagnetism with low energy consumption. BiFeO3, with features of relatively small bandgap and large polarization, provides an opportunity for investigating the optical modulation of magnetism. In this work, pure-phase BiFeO3 nanocrystals embedded in Al2O3 films are synthesized. It is demonstrated that the strain generated and accumulated during the growth process of BiFeO3 nanocrystals can lead to the modification of the atomic structure and thus produce a strain engineered bandgap. A distinguished light-modulated ferromagnetism is observed in BiFeO3 nanocrystals. Contributed by the strain engineered bandgap, the ferromagnetism of BiFeO3 nanocrystals can be modulated and enhanced more efficiently by light irradiation. It paves the way for modulating the ferromagnetic properties of BiFeO3 nanocrystals via bandgap engineering, which has promising applications in modern information technology.