In view of the few studies on the positive curvature of fiber cladding tube, a new Terahertz antiharmonic fiber based on the positive curvature regulation of fiber cladding tube is proposed. Firstly, the structure of Terahertz anti-resonant fiber with circular cladding tube and semi-circular semi-elliptical cladding tube is studied, and the influence of positive curvature on the limiting loss of fiber and the main mechanism are analyzed. Then, the structure of Terahertz antiharmonic fiber is optimized, and the two-layer cladding tube structure is adopted in the fiber gap. Finally, the loss limiting performance of three kinds of fiber structures is simulated, and the single-mode characteristics of the new Terahertz antiharmonic fiber are analyzed. The simulation results show that compared with other fiber structures, the new antiresonant THz fiber has a limited loss of 10-5 dB/m, and maintains good single-mode characteristics in 2.5~3.1 THz band.