A monocyclic nested hollow-core anti-resonant fiber (HC-ARF) with large 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 layer boundary conditions,and the influence of the structure parameters of HC-ARF on the transmission characteristics is analyzed.The simulation results show that the optimized fiber has the advantages of large transmission window,low confinement loss and flat dispersion.When the diameter 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.