Direct absorption methane digester can accomplish the coupled process of solar heat collection and photo and biochemical reaction of microorganisms simultaneously. In present work, a numerical model was developed to determine a tool to investigate the photo biochemical transformation process for developing engineering analyses, which is a coupled one of the simplified ADM1 model and the heat and mass transfer model. The model was validated by experimental data from two groups of different experiments. The effect of solar irradiance on the photo biochemical transformation process was also investigated by the model. The results show that a good agreement has been obtained between experimental data and simulations, which can be achieved in most of time. The study found that increasing solar irradiance is an effective method to improve the biogas production rate of direct absorption methane digester. With the solar irradiance increasing, the volume average temperature increases and the rates of biogas production, substrate reaction and total propionate all turn bigger. The effect of solar irradiance on biogas production rate is bigger than the one on the rate of propionic acid. In order to increase the heat storage of digester, the slurry surface evaporation of digester should be restrained.