The invention discloses a fragile armor-piercing shell W alloy and a preparation method thereof. The W alloy is prepared by mixing and sintering W powder, Ni powder, Fe powder and Re powder. The preparation method comprises the following steps: wet mixing; primary drying and screening; doping with wax; secondary drying and screening; shaping; dewaxing/pre-sintering; sintering; and heat treatment after sintering. The added Re powder serves as a more effective solution strengthening element, so that the density of the W alloy can be improved to a certain extent and the relatively high plasticitycan be kept; through the addition of Al2O3 powder of different grain sizes, the tensile strength can be reduced under the precondition that the dynamic compression strength and plasticity of the W alloy are guaranteed, and the fragility of the W alloy can be improved; and accordingly, fragments can be formed after the W alloy pierces armor, and then the purpose of secondary damage can be fulfilled.