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The crack propagation mechanism of Al_0.1CoCrFeNi high-entropy alloy (HEA) was investigated with the molecular dynamics method. The pre-crack propagation and stretching processes of single-crystal Al_0.1CoCrFeNi HEA and Al_0.1CoCrFeNi HEA with grain boundaries were simulated. The effects of strain rates and different crystal structures on the crack propagation of the alloy therein at room temperature were studied. They both exhibited plastic deformation and ductile fracturing, and the crack tip involved dislocations at 45° and 135° under the tensile stress. The dislocations formed in the intrinsic-stacking fault and stacking fault based on hexagonal closely packed structures spread and then accumulated near the grain boundary. At the position where hexagonal closely packed structures were accumulated, the dent was obviously serious at the 1/3 position of the alloy where the fracturing finally occurred. The yield strength for Al_0.1CoCrFeNi HEA with grain boundaries was lower than that of the single-crystal Al_0.1CoCrFeNi HEA. However, Young’s moduli for Al_0.1CoCrFeNi HEA with grain boundaries were higher than those of the single-crystal Al_0.1CoCrFeNi HEA. The grain boundaries can be used as the emission source of dislocations, and it is easier to form dislocations in the-single crystal Al_0.1CoCrFeNi HEA, but the existence of grain boundaries hinders the slippage of dislocations.