Abstract: The flexible peak regulation of gas-fired units increases the consumption of renewable energy but the resultant sharp gas-pressure fluctuations introduce new security risks to the natural gas network. This paper proposes a natural gas network dynamic security region to represent a set of peak regulation amounts of gas-fired units that satisfy dynamic pipe flow security constraints. First, impractical reasons for the dynamic security region with time-continuous gas loads' mass flow rates (MFRs) as the injection space are analyzed. To this end, the inlet MFR of each gas-fired unit is converted into a function of the active-power adjustment. And the dynamic security region is constructed by taking the discrete active-power adjustment as the injection space referring to the initial operating point. For balancing computational accuracy and efficiency, the continuous variables in the security region boundary search optimization model are discretized by the space-time orthogonal collocation. The gas network steady-state security region is revealed to be a reduced form of the dynamic security region, for it can be obtained by slacking some constraints in the optimization model. Simulation results show that the proposed method can overcome the over optimism of the gas network steady-state security region for intraday security analysis, thus enhancing coordinated situation awareness of electricity-gas coupling systems.