<p>A gas sensor based on a hexagonally organized core photonic crystal fiber (PCF) is presented in this research. One of the deadly and hazardous gases that contributes to environmental air pollution is hydrogen cyanide. This work presents the design of a novel PCF that provides minimal confinement loss and great sensitivity in the absorption frequency of hydrogen cyanide gas (HCN). With a hexagonal core and an outside cladding that has been filled with HCN gas, the suggested sensor is made of four layers of circular air holes in the cladding region. Version 5.4 of the COMSOL Multiphysics Software is utilized as a simulation and design tool. The findings are simulated using the finite element method (FEM). The result shows that at a frequency of 0.75 THz, the PCF provides a low confinement loss of zero for maximum input frequency and a high relative sensitivity of 91%. The effect of raising the HCN concentration on confinement loss and relative sensitivity is examined. Compared to existing sensors, the proposed PCF’s superior sensitivity and low confinement losses suggest that this optical structure could be a viable option for detecting this gas in both industrial and medical applications. We are certain that the sensor’s contribution to useful applications and its optimized geometrical structure will make it easy to manufacture. Additionally, our suggested PCF fiber will be perfect for a variety of businesses in the terahertz (THz) zones.</p>