In-seam horizontal boreholes are used to collect coalbed methane (CBM). Physical simulations of CBM drainage by in-seam horizontal boreholes are conducted. The evolution of gas pressure and permeability ratio indicate a rapid decline in a short time. As time goes by, the permeability ratio recovers, and the gas pressure declines slowly. In the borehole area, initially, the decline rates of the gas pressure and permeability ratio are positively related to the distance from the borehole, and the shape of the distribution of the gas pressures and permeability ratio is similar to that of multiple rings. In the non-borehole area, the decline rates of the gas pressures and permeability ratio is almost the same at different points. When the borehole spacing is optimum, the interaction effect accelerates the drainage of CBM. The permeability is initially controlled by effective stress and is mainly affected by matrix shrinkage after a period of time. (C) 2019 Elsevier B.V. All rights reserved.