It is considered that the gas pre-extraction measure in coal bed is one of the most effective methods to prevent the gas disaster. But, gas drainage hole is usually deep and its conditions are poor, it is difficult to use conventional methods to examine effective length and inside condition. In order to measure the parameters of gas drainage hole quickly and exactly, a new detecting device for gas drainage hole has been developed, which can walk in the hole and has measurement and survey functions. The geometric characteristics of hole, coal rock images and gas density are directly measured. These parameters are the significant evidences for optimizing drilling parameters, improving drilling quality and ensuring the effect of gas drainage.