This work presents an analysis of the dynamic behaviour of a HRSG (heat recovery steam generator) during start-up. A calculation program based on a quasi-steady method is constructed A typical high-pressure HRSG is designed conceptually and analysis is performed to examine the influence of the gas inlet condition of the HRSG on its start-up behaviour. Effects of the gas turbine operation mode and the gas bypass are analysed. In addition, the water level control during start-up, which is one of the most important facts in the rear plant operation, is simulated. Through a parametric calculation, the effect of the control parameters on the start-up behaviour is analysed and examples of optimum control are demonstrated.