In this paper, we report the design and simulation of a bias-selectable dual-band photodetector operating in the visible (VIS) and near infrared (NIR) regions. The photodetector consists of two back-to-back avalanche photodiodes (APDs) with InGaAs and Si absorption layers respectively. The structure and electrical and optical properties of the dual-color photodetector were designed and simulated by exploiting Silvaco software. The results obtained on the basis of numerical simulation include the current-voltage, capacitance-voltage, spectral response, etc. The optical simulation shows the detection capability in the VIS and NIR ranges, cut-off wavelengths of 1.0 µm and 1.8 µm depending on the applied bias polarity. Comparing with using the PIN structure as element device, the dual-band photodetector based on the APD configuration could detect the very weak signal, realizing few photons, even single photon detection.