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As a piezoelectric material, K0.5Na0.5NbO3 (KNN) has broad application prospects in ultrasonic transducers, sensors, and biomedicine areas. Its structure information under high pressures is of great significance for guiding device design. In this study, the high-pressure structural evolution of KNN has been studied. Two structural phase transitions were revealed by high-pressure Raman spectrum. The phase transition boundary was found by Raman vibration mode analysis, with transformation ranges of 2.5–4.6 and 6.8–9.4 GPa. The phase structures were determined by in situ neutron diffraction, with a phase transformation path of orthogonal Amm2 (O) → tetragonal P4mm (T) → cubic Pm3¯m (C) structure at high pressures. Synchrotron x-ray diffraction further confirmed the phase transformation path. During the processes of phase transitions, the path of Nb atom was clearly described as moving toward [1¯01] and then [100] direction. An output power density of KNN ceramic devices was comparable to that of commercially available PZT 95/5. The density of KNN ceramic is approximately half that of PZT 95/5, which means a significant advantage in terms of weight reduction and miniaturization of equipment in global demand. The phase transition of ferroelectric materials under high pressures provides scientific guidance for the development of high-power pulse power devices.