In this work, a Fabry–Perot (F–P) antenna based on metal integrated suspended lines (MISLs) at the K-band for microwave wireless power transmission (MWPT) is proposed. The antenna’s contribution lies in its adaptation of the MISL structure and its all-metal design, which achieves low loss, high gain, and high-power capability. The entire antenna structure is dielectric-free, further reducing apparent dielectric loss at high frequencies. Meanwhile, the radiation structure is surrounded by a metallic wall to minimize radiation loss. A metal partially reflective surface (PRS) on the top of the antenna, together with a metal ground plane, constitutes an air-filled resonant cavity. The reflection and transmission of electromagnetic waves in the PRS are effectively controlled to be in phase, thereby enhancing its gain by optimizing the PRS and resonant cavity dimensions. A simple slot antenna is employed as the primary source for the F–P resonant cavity. The antenna is processed layer by layer and then assembled to lower machining costs and complexity. Experimental results indicate that the proposed F–P antenna achieves an aperture efficiency over 60% and a measured peak gain of 18.4 dBi at 23.85 GHz with an aperture size of 2.86 λ₀ × 2.86 λ₀.