Tuning the charge transfer processes through a built-in electric field is an effective way to accelerate the dynamics of electro- and photocatalytic reactions. However, the coupling of the built-in electric field of p–n heterojunctions and the microstrain-induced polarization on the impact of piezocatalysis has not been fully explored. Herein, we demonstrate the role of the built-in electric field of p-type BiOI/n-type BiVO₄ heterojunctions in enhancing their piezocatalytic behaviors. The highly crystalline p–n heterojunction is synthesized by using a coprecipitation method under ambient aqueous conditions. Under ultrasonic irradiation in water exposed to air, the p–n heterojunctions exhibit significantly higher production rates of reactive species (·OH, ·O₂^–, and ¹O₂) as compared to isolated BiVO₄ and BiOI. Also, the piezocatalytic rate of H₂O₂ production with the BiOI/BiVO₄ heterojunction reaches 480 μmol g^–1 h^–1, which is 1.6- and 12-fold higher than those of BiVO₄ and BiOI, respectively. Furthermore, the p–n heterojunction maintains a highly stable H₂O₂ production rate under ultrasonic irradiation for up to 5 h. The results from the experiments and equation-driven simulations of the strain and piezoelectric potential distributions indicate that the piezocatalytic reactivity of the p–n heterojunction resulted from the polarization intensity induced by periodic ultrasound, which is enhanced by the built-in electric field of the p–n heterojunctions. This study provides new insights into the design of piezocatalysts and opens up new prospects for applications in medicine, environmental remediation, and sonochemical sensors.