In this paper, the dark electrical properties are studied by measuring the dark current-voltage characteristics of a type of domestic single-junction (SJ) GaAs/Ge solar cell after proton irradiation. Using a double exponential mode for the dark electrical properties of p-n junction, the dark I-V curves of the proton-irradiated SJ cells are mathematically fitted, and there are four kinds of typical parameters, namely serious resistance (Rs), parallel resistance (Rsh), diffusion current (Is1), and recombination current (Is2), which are determined to characterize the irradiation effects. Hence, four parameters such as Rs, Rsh, Is1 and Is2 are significantly changed after proton irradiation, where Rs, Rsh, Is1 increase while Rsh decreases with increasing the displacement damage dose. In addition, R_{s } increases with displacement damage dose, which is unrelated to proton energies. Theoretical analysis indicates that the above-mentioned changes of the parameters result from the damage distributions in different regions of the solar cells. Irradiation-induced damage in the base and emitter regions of the cells could induce Rs and Is1 to augment, while junction-region damage causes the Rsh to decrease but the Is2 to increase.