Abstract: According to the characteristics of steam generator operating under supercritical pressure, the model of supercritical once-through steam generator consisted of helical coils used in HTGR was improved to study the dynamic characteristics of the supercritical steam generator, based on the optimized model and heat characteristics analyses in steady state conditions. Two scenarios, the helium flow rate jump and successive jumps of helium and water flow rates, were simulated to analyze the dynamic responses of various temperatures and heat transfer parameters. The simulation results indicated that the helium flow rate step change had influence on both primary and secondary side parameters. Yet, the secondary side parameters changed more slowly due to the delay effect caused by large heat capacity of metal and water. In the test of successive jumps of helium and water flow rates, some parameters had slight fluctuation, which was attributed to the heat transfer characteristics of supercritical water.