The invention discloses a high-density ultrahigh-strength tungsten-cobalt heat-resistant alloy and a preparation method thereof and belongs to the technical field of heat-resistant alloys. The alloy comprises the following chemical components in percentage by weight: 0-20% of Ni, 10-35% of W, 0-3% of Ti, 0-3% of Al, 0-8% of Nb and the balance of Co and other inevitable impurity elements and micro elements such as rare earth. The preparation method of the high-density ultrahigh-strength tungsten-cobalt heat-resistant alloy comprises the steps of vacuum sensing, vacuum and consumable re-melting or re-melting electroslag. Compared with the prior art, the high-density ultrahigh-strength tungsten-cobalt heat-resistant alloy is excellent in comprehensive performance, high in density, high in toughness and ultrahigh in strength; the density reaches 9.0-11.0g/cm<3>; the impact toughness is not less than 100J/cm<2>; the tensile strength is not less than 1400MPa.