In order to improve the contrast of the half adder, a high contrast Terahertz wave half adder design scheme based on mode interference is proposed. In this scheme, a linear waveguide is introduced into the complete two-dimensional square lattice tellurium dielectric pillar photonic crystal, and the logic operation function is realized. A comprehensive performance analysis of the half adder is conducted by combining the plane wave expansion method (PWM) and the finite difference time-domain method (FDTD) in Rsoft software, and optimization is carried out for the position of the incident waveguide and the offset of the dielectric column. The simulation results show that the designed half adder can perform two input half adder logic operations in the 2.85 THz band. The contrast between the sum end and carry end of the half adder can reach 21.37 dB and 5.96 dB, respectively, with an overall response time of 25.2 ps. Theoretically, it can achieve a data transmission rate of 39.68 Gb/s. The structure of the half adder is simple and compact, with a size of only 0.84 mm×0.72 mm.