Abstract: Determining the venting time of a gas trunk pipeline segment provides an important basis for formulating an emergency plan in the advent of unexpected accidents. As the natural gas venting process corresponds to the transient flow, it is necessary to establish a transient hydraulic-thermal simulation model in order to determine the venting time. In this paper, based on two kinds of venting scenarios in which there is only one venting point in the venting system of a gas trunk pipeline segment—namely, where the venting point is either at one of the two ends or at the junction of two gas trunk pipeline segments—transient hydraulic-thermal simulation models are established. The models consist of gas flow governing equations, the gas state equation, gas physical property equations, initial conditions, and appropriate boundary conditions. The implicit central difference method is used to discretize the gas flow partial differential equations, and the trust-region-dogleg algorithm is used to solve the equations corresponding to each time step, in order to dynamically simulate the whole venting process. The judgment condition for the end of the venting process is that the average pressure of gas trunk pipeline segment is less than 0.11 MPa (actual pressure). Comparing the simulation results of the proposed model with those of the OLGA software and real operational data, we find that the venting time error is less than 10%. On this basis, a venting valve opening control principle is proposed, which prevents the venting noise from exceeded the specified noise value (85 dB) in the venting design of domestic gas pipeline projects. The established calculation model for venting time (dynamic simulation model) for a gas trunk pipeline segment and the proposed opening control principle of venting valve provide reference for the optimal operation of gas pipeline venting systems.