The embodiment of the invention provides a building method and device for a homopolar crossing fault model between DC systems, and relates to the field of a power system. By the building method and device, an equivalent model of the homopolar crossing earth-free fault between the DC systems can be built. The building method comprises the steps of acquiring a homopolar crossing earth-free fault model by a David equivalent value; simulating the earth-free fault model to obtain a first network topological map and a second network topological map; building a first boundary equation set and a second boundary equation set according to the first network topological map, the second network topological map and a preset rule; acquiring trigger angle values of conversion stations in the first networktopological map and the second network topological map according to a simulation result of the earth-free fault model; building a homopolar crossing earth-free fault equivalent model; and calculatinga fault equivalent impedance value of the homopolar crossing earth-free fault equivalent model according to the trigger angle values of the conversion stations in the first network topological map and the second network topological map, the first boundary equation set, the second boundary equation set and an equation set between a preset rectification side and an inversion side.