A model for multiple-quantum well (MQW) p-i-n photodiode (PD) is presented. The model accounts for the responsivity spectrum and the polarization-dependent loss. The important physical effects for modeling MQW PDs are the carrier transit time through the MQW layers and the free carrier density in the quantum wells. Furthermore, both effects influence the internal quantum efficiency and saturation of the optical absorption. The model is verified by comparing with the measurement results of a waveguide integrated MQW p-i-n PD. It is suitable for optimizing the design and describing the properties of MQW p-i-n PDs.