The investigation of the single-photon properties of a wafer-bonded Ge/Si single-photon avalanche photodiode (SPAD) is theoretically conducted. We focus on the effect of the natural GeO layer (hydrophilic reaction) at the Ge/Si wafer-bonded interface on dark count characteristics and single-photon response. It is found that the wafer-bonded Ge/Si SPAD exhibits very low dark current at 250 K due to the absence of threading dislocation (TD) in the Ge layer. Owing to the increase of the unit-gain bias applied on the SPAD, the primary dark current (I ) increases with the increase in GeOthickness. Furthermore, the dependence of the linear-mode gain and 3 dB bandwidth (BW) for the dark count on GeO thickness is also presented. It is observed that the dark count probability of the Ge/Si SPAD significantly increases with the increase in GeO thickness due to the increase of the I and the reduction of the 3 dB BW. It is also found that with the increase in GeO thickness, the external quantum efficiency, which affects the single-photon detection efficiency (SPDE), drastically decreases because of the blocking effect of the GeO layer and the serious recombination at the wafer-bonded Ge/Si interface. The afterpulsing probability (AP) shows an abnormal behavior with GeO thickness. This results from the decrease in avalanche charge and increase in effective transit time.