A monocyclic nested hollow-core anti-resonant fiber (HC-ARF) with l arge transmission window and low confinement loss is proposed.The transmission characteristics of HC-ARF are simulated by full-vector finite element method combined with perfectly matched l ayer boundary conditions,and the influence of the structure parameters of HC-ARF on the tran smission characteristics is analyzed.The simulation results show that the optimized fibe r has the advantages of large transmission window,low confinement loss and flat dispersion.When the di ameter of the fiber core is 50 μm,the number of the anti-resonant tube is 6,the thickness of the anti-resonant tube t is 0.30 μm,the diameter of the outer anti-resonant tube d is 32.50 μm,and the diameter of the inner anti-resonant tube d₁ is 21.13 μm,the confinement loss is lower than 0.21 dB/ km and the dispersion value is (1.1±0.3) ps/(nm·km) in the wavelength range of 1 260 —1 675 nm,the confinement loss at 1 550 nm is 0.078 dB/km.