The startup of anaerobic reactors is a very important issue for their application in the treatment of wastewaters and solid wastes. However, very limited information is available about the quantitative and dynamic modeling of anaerobic reactor startup in the literature. In this work, by introducing a linear phenomenological equation into the IWA Anaerobic Digestion Model No. 1 (ADM1), we developed a model to describe the dynamic behavior of the startup of anaerobic reactors. The model was calibrated and verified using the experimental results of two anaerobic reactors: an upflow anaerobic sludge blanket (UASB) reactor fed with soybean processing wastewater and a continuously stirred tank reactor (CSTR) system with simulated municipal solid wastes as the substrate. The simulation results matched with the experimental data well, and the model could capture the variation trends of the components. This agreement demonstrates the rationality and validity of the established model for describing the startup of anaerobic systems.