The Ge-based photodetector has a broad application prospect in light detection with its unique communication bandwidth response characteristics and good compatibility with CMOS technology. However, the response band of current commercial detectors is generally limited to a specific band, which is not easy to meet the detection requirements of multiband fusion and miniaturization. Therefore, in this paper, a graphenemetal-insulator-semiconductor (MIS) junction-based photodetector is fabricated by introducing a thin SiO₂ interface layer between multilayer graphene and N-type Ge. The effect of SiO₂ with different thicknesses and graphene layers in the MIS junction device is investigated. The influence of the number of layers on performance of the MIS junction device is also tested. The spectral response range, current-voltage curve, responsivity, on-off ratio, and other optoelectronic properties of the device are tested. The results show that the device has a response in the wavelength range of 254-2200 nm, and the responsivity and the on-off ratio peaked at 980 nm, which are 73.86 mA/W and 1.74 × 10³, respectively. The rise and fall times are 1 ms and 3 ms, respectively.