Felsic gneiss is a major type of ultrahigh-pressure metamorphic rock in continental subduction zones. As an important mineral of felsic gneiss, the phase behavior of K-feldspar is of great significance for studying subducting seismic activities, crust-mantle interactions, and plate tectonics in the Earth’s interior. In this study, we investigated the phase relations in K-feldspar using Raman spectroscopy combined with externally heating diamond-anvil cell (DAC) under high-pressure (28 GPa), high-temperature (400 °C), and simultaneous high pressure-temperature (P-T) (14 GPa, 430 °C) conditions. The phase diagram of K-feldspar at relatively low temperatures has been constructed, and together with previous results, a more comprehensive P-T phase diagram of K-feldspar is established. K-feldspar undergoes a reversible phase transition from C1 to P1 (metastable K-feldspar) symmetry at 10.3 GPa. Metastable K-feldspar is an intermediate phase from K-feldspar to K-holl-I (KAlSi₃O₈ with a hollandite-I structure) in extremely cold subduction slabs (<2 °C/km) or the center of some old, cold, and rapidly subducting slabs. Metastable K-feldspar is stable even at 11.4 GPa and 400 °C in enriched hydroxyl group (OH) environments, which shows greater high-P-T stability than K-feldspar. Thus metastable K-feldspar could withstand subduction-zone fluids, low-temperature metamorphism, and survive to deeper than previously expected. These results enhance our understanding of the formation paths and conditions of K-holl-I, the subducted depth of K-feldspar, the effect of subduction-zone fluids on continental subduction, and provide a possible reason for the origin of intermediate- and deep-focused seismicity.