Hydraulic flushing technology is one of the most widely used pressure relief and permeability improvement measures and it works well in soft and low permeability coal seams. The physical simulation of hydraulic flushing was determined based on self-developed hydraulic flushing test system, and two CBM drainage tests were conducted before and after hydraulic flushing in order to evaluate its pressure relief and permeability improvement. Equivalent drainage radius and effective drainage time were defined to evaluate the hydraulic flushing technology quantitatively. As a conclusion, the equivalent drainage radius increased to 1.7 times and the effective drainage time reduced to 56% after hydraulic flushing. The results indicate that hydraulic flushing can improve permeability of coal seam and drainage efficiency greatly by increasing the porosity and fissure of coal seam. Therefore, the key of hydraulic flushing technology is how to improve the porosity and fissure of coal seam effectively and stablely.