We propose and experimentally demonstrate a highly sensitive gas pressure sensor in parallel configuration based on harmonic Vernier effect. The sensor consists of two segments of 75 μm hollow-core capillaries as the sensing and reference Fabry Perot cavity. An open passage is formed by a 5 μm capillary in the sensing cavity while the reference cavity is sealed by a single-mode fiber. A pressure sensitivity of 279.52 pm/kPa is achieved. Our scheme uses "air" instead of "silica" reference cavity, which allows higher sensitivity for a given accuracy of cavity length control. In addition, we also developed a "superposition envelope" method, which improves the contrast by at least 3 dB for the Vernier envelope. This design for high-sensitivity optic fiber gas sensor can be a good candidate for ultrasensitive gas pressure applications. © 2022 Elsevier Ltd