A novel pressure sensor based on a high-aspect-ratio structure formed by SU-8 photoresist is proposed. The advantage of the structure lies mainly in the design of separation of sensing membrane deformation with the length change of Fabry-Perot (F-P) cavity, which restricts the linear measurement in most traditional pressure sensors. The sensor is based on F-P interferometer and phase demodulation method. The structural parameters have been ascertained, which will be dealt with during sensor fabrication through numerical simulation. The main sensing elements of the designed model include a silicon membrane and a square column on it. The membrane is with diaphragm thickness of 100 μm and radius of 1500 μm. Meanwhile, the square column is with the dimension of 100 μm×100 μm×400 μm. The sensitivity of the structure model is presented as 2.606 μm/MPa from simulation. Under the guidance of the simulations, the fundamental processing is defined.