We presented a decagonal cladding and hexahedron core-based photonic crystal fiber (PCF) to sense chemicals in the terahertz frequency (THz). Circular air holes (CAHs) in the cladding region make up the proposed sensor. A wide variety of frequencies were evaluated to analyze the sensor’s performance in terms of sensitivity, confinement loss, and effective material loss respectively. We designed and quantitatively analyzed the optical properties of our proposed hexahedron-based PCF sensor using the finite element method (FEM). Square-shaped air hole length, strut, and core size have also been researched to improve the performance of the proposed sensor’s sensing components and fabrication tolerance. At ideal conditions, the suggested PCF sensor has a maximum relative sensitivity of 94.65%, confinement loss of 6.01 × 10− 8 cm− 1 , effective material loss (EML) of 9.16 × 10− 4 cm− 1 , and effective mode area (EMA) of 1.35 × 10− 7 m2 . We are confident that the suggested sensor’s optimized geometrical structure will be manufacturing-friendly, as well as the sensor’s contribution to practical uses. Furthermore, our proposed PCF fiber will be ideal in the terahertz (THz) regions for various optical communication applications and medicinal signals.