Abstract: At present, the injection/fall off test is widely used for the permeability of coal bed methane wells in China.Through open hole test and casing test of test wells, it is found that the permeability from casing test is 9.82 times of that from open hole test, which shows that the permeability from open hole test is obviously low.The main cause for it is that the injection and shut in time is short, and the radial flow doesn't appear.Because most coal bed methane blocks of China are in exploration phase, and the open hole injection/fall off test is usually adopted, comparison between open hole and casing injection/fall off tests can obtain the injection time and shut in time of the typical well of a block, and improve the accuracy of permeability interpretation.