The single-mode fiber was heated tapering by flame brush method, forming an optical waveguide structure which has a core radius of micro-nanometer level, and the low-loss optical fiber tapered was optimized by the different coefficients of thermal insulation to meet insulation standards. The tapered optical fiber with the best shape was designed, whose constant cone angle is 2 mrad, wavelength is 400 μm and fiber radius is 4 μm. Matlab simulation results show that, the transmission perspective of tapered optical fiber with length of 23 mm is 99.7%, the radiation resistance of tapered optical fiber with length of 63mm is 99.6%. Since the power is coupled to the high-order mode, the loss is sufficiently suppressed, so that the optical coupling transmission of the tapered fiber with a constant taper angle can be achieved without sacrificing the quality of the transmission.