A wireless passive flexible pressure sensor integrating ultra-high frequency (UHF) radio frequency identification (RFID) technology with capacitive sensing principles was designed and fabricated to address the sensing requirements of flexible electronics and the Internet of Things. The sensor combines a conventional RFID tag with a capacitive unit to enable real-time pressure monitoring. The response mechanism linking external pressure to the phase variation of the RFID signal was elucidated: under applied load, the flexible dielectric layer undergoes compressive deformation, reducing the spacing between conductive layers and thus altering the capacitance of the sensing element. This capacitance change modifies the reflection boundary condition at the antenna port, manifested as a pronounced phase shift in the reader-received signal. To optimize performance, the influence of a micro-pyramidal structured dielectric layer on sensitivity was experimentally investigated. Performance tests demonstrate that the developed sensor exhibits excellent response characteristics in the 0~5 kPa range, with a sensitivity of 4.011°/kPa and an upper detection limit of 20 kPa, evidencing a broad linear detection range and good stability. This work provides a reference scheme for wireless real-time pressure detection in the UHF band and promotes the potential application of wireless passive pressure sensors in the field of wearable electronic devices.