An open-cavity optical fiber gas pressure sensor based on the Vernier effect is proposed. The sensor consists of two Fabry-Perot interferometers (FPIs) connected in parallel. As a sensing FPI, FPI-1 is an open-cavity FPI and composed of single-mode fiber (SMF), hollow core capillary (HCC), and twin-hole and dual-core fiber (THDCF). The open-cavity FPI structure is constructed using the air holes of THDCF. FPI-2 consists of SMF and HCC with a large inner diameter and works as a reference FPI. The manufacturing cost of the sensor is low and easy to manufacture. This study explored the relationship between the magnification factor and the cavity lengths of the sensing FPI and the referencing FPI. After optimizing, the sensor has a sensitivity of -168.82 nm/MPa in the gas pressure range of 0.1-1.6 MPa, which is 48.37 times the sensing sensitivity of single FPI. In addition, the temperature crosstalk of the sensor is 2.285 x 10(-4) MPa/degrees C. Repeated experiments have demonstrated that the sensor has good repeatability and stability. Therefore, the proposed sensor structure provides a new idea for the design and application of optical fiber gas pressure sensors with ultrahigh sensitivity and lower temperature cross sensitivity.