Refractive index sensing is attracting extensive attention in biochemical sensing using terahertz technology. Various structures with strong confinements have been used to design sensors for improving the interaction between the terahertz wave field and the analytes, such as photonic crystals, nanowires, plasmonic structures, and metamaterials. Terahertz wave sensors based on two-dimensional photonic crystal have been used in various areas ranging from disease diagnostics to environmental pollution detection. For improving the performance of terahertz sensor, a sensing scheme based on high-density polyethylene sunflower-typecircular photonic crystal structure is proposed. The designed sensor contains two symmetrical sample cells surrounding a cavity in a circular photonic crystal. The transmission properties of the terahertz wave sensor are analyzed based on COMSOL Multiphysics when the central sample cells are filled with analyte with different refractive indices. The sensor characteristics depending on the structure parameters are analyzed. The choice of these parameters is discussed. Finally, a sensitivity of 10.4 μm/RIU, Q-factor of 62.21, and figure-of-merit of 1.46 are realized. The results in this work are expected to be able to extend the circular photonic crystal-based sensor to terahertz wave region.