As a kind of excellent multifunctional metal oxide semiconductor, KxNa1-xNbO3 (KNN) has been widely applied in a variety of fields such as photocatalysis and energy harvesting due to its excellent piezoelectric, dielectric and photovoltaic properties in recent decades. In this report, octahedron-shaped K0.4Na0.6NbO3 (KNN-6) microstructures assembled by cubic nanoparticles with {010} exposed facets were synthesized via a one-pot hydrothermal reaction. Due to the accumulation of electrons on the exposed facets, which was conducive to the separation of photo-generated electron-hole pairs, the microstructures could achieve a highly efficient photocatalytic performance for wastewater degradation. In addition, owing to the piezoelectric effect of KNN crystals, the degradation efficiency could be further enhanced by introducing ultrasonic vibration. Using methylene blue (MB) as the organic dye to evaluate their wastewater degradation efficiency, the KNN microstructures achieved their best catalytic performance when the atomic ratio of KOH to NaOH in the reactant was set at 4 : 6 (KNN-6). Under the synergistic effect of light irradiation and ultrasonic vibration, MB could almost be completely (99%) degraded within 40 minutes by KNN-6 microstructures, which was several times more efficient than that of pure NaNbO3 or KNbO3 in previous reports. This work demonstrated that the K0.4Na0.6NbO3 (KNN-6) microstructure could be a prominent candidate for wastewater purification. The formation mechanism of KNN crystals and the role of the piezoelectric effect in the photocatalytic process were also discussed.