In order to realize the strict dual-mode transmission and large mode field area of the multi-core optical fiber,the multi-core fiber is introduced into the air hol e and designed into an octagonal structure.First,The electromagnetic field model of the fiber by using COMSOL software is established,and then the finite element method is used to systematically analyze the influence of the three structural parameters of the relative aperture size,the core-clad refrac tive index difference and the core spacing on the fiber mode characteristics and the effective mode fi eld area.The bending loss of the optical fiber is discussed under different bending radii in the end. According to the analysis results and combined with the normalized frequency constant to find sui table structural parameters,the fundamental mode field area of the fiber can reach 1730μm²ⁱⁿ the flat state. When the bending radius is greater than 0.45m,the bending loss is less than 10－3 dB/ m,the area of the fundamental mode field can still reach 1685μm^2at this time.This optical fiber maintains few-mode transmission and achieves a large mode field area and low bending loss ,so it has broad application prospects in large-capacity and high-power optical fiber transmiss ion systems.