Due to excellent piezoelectric properties and electromechanical coupling properties, lead-based piezoelectric ceramics represented by lead zirconate titanate Pb(Zr_xTi_1–x)O₃ (PZT) are widely used in science and technology, industry, military and daily life. However, the content of Pb in PZT-based ceramics exceeds 60% (mass ratio), which will cause serious damage to human ecological environment in the process of their production, use and waste treatment. Therefore, the development of lead-free piezoelectric ceramics has become one of the hot research spots. Potassium sodium niobate (K_0.5Na_0.5)NbO₃ (KNN) lead-free piezoelectric ceramics are considered as one of the most promising material systems to substitute for lead-based piezoelectric ceramics because of their good piezoelectric properties and higher Curie temperature. Through many years of researches, the piezoelectric properties of modified KNN based lead-free piezoelectric ceramics have approached to or even exceeded those of some lead-based piezoelectric ceramics. Combining with our relevant work, we comprehensively review the research progress of high piezoelectric activity of KNN based lead-free piezoelectric ceramics, especially focus on the research progress of high-performance potassium sodium niobate lead-free piezoelectric ceramics, preparation technology and related theoretical mechanisms. The future research direction and prospect of KNN-based lead-free piezoelectric ceramics are also presented.