An octagonal cladding with a hexahedron core PCF sensor is designed to detect chemical compounds at terahertz frequency. The proposed sensor contains circular air holes (CAHs) in the vestibule area. To measure the performance of the sensor effective material loss, confinement loss, and a large frequency area have been used in terms of relative sensitivity (RS). For the design and analysis of our proposed hexahedron PCF sensors, the finite element (FEM) utilizes numerally the optic parameters. The diameter and core size of the proposed air whole were checked to optimize the production tolerance and sensing performance. At the area of 1 THz spectrum Ethanol, Benzene, and Water are used for numerical examination The fiber appears to be a high Relative sensitivity which is 91.25 %, 93.80 %, and 89.95 % respectively for Ethanol (n = 1.354), Benzene (n = 1.366) and Water (n = 1.330). The sensor provides low confinement losses with 5.95 × 10−08 dB/m, 6.25 × 10−08 dB/m, and 5.94 × 10−08 dB/m corresponding for those three chemicals. The geometric construction of the projected sensor can be used in chemical sensing in biomedical, industrial quality control, material research, clinics, gas sensors, micro-optics, and many communication applications in THz technology.