A ground-resistance detection device, which is coupled to a power system including a first power line, a second power line, and a protective earth, includes a ground-resistance detection circuit and a controller. The ground-resistance detection circuit includes a first input node and a second input node. The first input node is coupled to either the first power line or the second power line. The second input node is coupled to a ground terminal. The ground-resistance detection circuit generates a DC output voltage according to the voltage of the first input node, the voltage of the second input node, and the DC reference voltage. The controller determines the ground resistance between the protective earth and the ground terminal according to the DC reference voltage and the DC output voltage.