The invention relates to an underground gas pressure testing technology, in particular to a device for testing the length of a water column in drilling a hole for measuring the gas pressure, wherein apipe wall of the front section of a steel pipe is provided with a plurality of distributed holes so that a pressure-measuring embossing pipe is formed at the front section of the steel pipe, a stainless steel wire is arranged in the center of the pressure-measuring steel pipe, the pressure-measuring steel pipe and the stainless steel wire are spaced, and the end surface between the front end of the pressure-measuring steel pipe and the front end of the stainless steel wire is sealed by an insulating sleeve; the tail end of the pressure-measuring steel pipe is sealed, a precision pressure gauge is arranged at the tail end of the pressure-measuring steel pipe, and the pressure-measuring steel pipe is used for detecting the total pressure of gas and liquid in a pressure measuring drilling hole; a first electrode of a direct current power supply is connected with the pressure measuring steel pipe through a test wire through and a variable resistance box, and a second electrode is connected with the stainless steel wire through the test wire. According to the invention, the method accurately corrects the water pressure of a water-containing drilling hole. The device and the operation of the method are simple, the calculation is accurate, and the testing circulation is extremely short. Gas over limit or impact accident caused by a prior technology are avoided.