In order to improve the transmission capacity of the fiber and obtain better transmission performance, a six-core fiber with low crosstalk and large mode area assisted by three-layer grooves is designed, each core of the multi-core optical fiber is wrapped by three layers of low refractive index grooves, and a clapboard structure is added in the cladding. The effects of core radius, cladding index, groove depth and groove width on inter-core crosstalk and mode field area were studied by simulation with COMSOL and Rsoft software. The results show that the inter-core crosstalk can reach -55.429dB and the mode field area can reach 305.061μm2 when the wavelength is 1550nm, the core index is 1.4457 and the fiber radius is 40μm. Low crosstalk and large mode area can be obtained in multi-core optical fibers with this structure.