Abstract: Conventional DST tester is inadaptable to CBM well, in order to solve the problem and to avoid the wrong operation, through research on DST theory and analysis of the properties of coal reservoir, special DST equipment composed of external inflatable packer and downhole electromagnetic switching valve was developed, and was used to conduct DST in target seams. The result shows that the equipment is simple in structure, convenient for operation, the testing results are reliable, duration is short and cost is low, suitable to test seams in open well and cased well.