A silicon-based germanium waveguide photodetector was demonstrated and its reliability related items were investigated. For different reverse biases, the slopes of the dark current increment versus stress time curves were first found to be the same, which made the lifetime extrapolation feasible. The lifetime of the photodetector under different bias was predicted by using a simple extrapolation method. In order to maintain the 10-year lifetime of the photodetector, the bias voltage should be kept lower than −3 V . For the first time, the degradation mechanism under stress biases was analyzed in detail by the reaction-diffusion (RD) model. The experimental results agree well with the theoretical derivation based on RD model.