In this study a computer program. HaGuo Start-up Simulation Program (HGSSP) for thermal-hydranlic analysis is developed for simulation of star-up behavior of a once-through bolier equipped with various integral separator start-up systems. With the simulation results, design principles for optimizing a start-up system for design and optimization of start-up procedures for operation, which will result in minimum start-up losses by reducing the start-up time as much as possible, can be obtained. On the basis of Dolezal's model, an improved analytical-numerical method is proposed to solve the coupled energy equations for flue gas and working fluid describing the transient hear transfer ocurrig in individnal heation surfaces. In order to verify the program presented here, the cold start of a 600-MW supercritical once-through boiler located in China on July 6, 1993. is sinudated and a comparison with measured adta made. This comparison indicates reasonable agreement between the computational main steam pressure, water separator level, and drain flow. Economizer inter and outlet temperatures. Economizer pressure, and final superheater inlet temperature and those measured. Finally, the standard cold and hot start-up processes of a Harbin Boiler Works supercritical once-through boiler are simulated with HGSSP to acquire the start-up curves and suitable drain valve dimensions corresponding to a predetermined pressure-raising curve.