By combined martensitic transformation calculation based on phenomenological theory and in-situ tensile tests, it was demonstrated that the mechanical properties of polycrystalline Cu–Ni–Al alloys can be enhanced by changing the stress state and improving the strength of grain boundaries. The experimental results indicated that, compared with ordinary hot-rolled Cu–Al–Ni alloys, controlled grain growth by continuous unidirectional solidification can change the stress state of grain boundaries and restrain intergranular cracks caused by local stress concentration, thereby making a significant tensile strain increase of 21.3% and superelasticity strain increase of 15.2%. Minor addition of B can enhance the bonding strength of grain boundary and reduce the formation possibility of intergranular cracks, resulting in a maximum tensile strain and superelasticity strain increase to be 3.1% and 2.0% respectively. These results would provide a theoretical and experimental foundation for enhancing mechanical properties of polycrystalline Cu–Al–Ni alloys by grain boundary control.