An easy approach is suggested to obtain excellent piezoelectric performances in potassium sodium niobate (KNN)-based ceramics simultaneously with low dielectric loss (tanδ), high Curie temperature (TC), and electromechanical coupling factor (kp). Herein, a KNN-based ceramics system with nonstoichiometric Nb5+ is designed. Excessive Nb5+ occupying the B-site significantly influences the microstructural features and electrical properties of KNN-based ceramics. Furthermore, the excessive Nb5+ improves the temperature stability of ceramics by providing the domain wall pegging effect and defect dipole. A high TC = 300 °C, large kp = 0.516, and d33 = 450 pC/N can be simultaneously obtained in the KNN-based ceramics with nonstoichiometric Nb5+. These results confirm that the comprehensive electrical properties of KNN-based ceramics can be tuned by optimizing the content of nonstoichiometric Nb5+.