Abstract: The conventional inspection devices cannot be used in gas pipelines with high flow velocity condition. To solve this problem, a method to control actively the running speed of the devices by drilling the discharge hole was proposed, a speed control model based on discharge hole was established, and the discharge formula was derived. But this formula is inconvenient for application for its local resistance coefficient of discharge hole cannot be determined easily. According to the characteristics of the discharge hole in the inspection device and the theory of fluid mechanics on the orifice outflow, a discharge model for pipeline inspection device based on thin-wall orifice outflow was established, and the formula of thinwall orifice outflow was derived. It is shown that the local resistance coefficient of discharge hole is 0.06 when the Reynolds number (Re) of medium transportation is not lower than 105 and the discharge hole is small. And the newly built discharge model was used to predict the running speed of the inspection device with fixed discharge area and that of the speed control pig, and the calculation results are in accordance with the actual operation. To sum up, this new discharge model is structurally simple with fewer parameters and convenient for the industrial field calculation.