Modular HTGR nuclear power plant because of inherent safety and high thermal efficiency shows good prospects for development. The current high-temperature reactor demonstration power plant (HTR-PM) using two thermal power of 250MW of modular HTGR with an electric power 211MWe turbine unit. As one development goals of multi-reactor with one turbine unit, millions of kilowatt nuclear power plant will use more reactor module and steam generator module more like demonstration power plant (HTR-PM) with 1000MWe supercritical turbine generator unit. spiral-style super-critical steam generator design, modeling is a key factor. Analyzing the structure and the characteristic of moderate spiral coil steam generator which is used in Modular HTGR demonstration power plant, from the mechanism of equipments, based on the law of quality conservation, energy conservation, momentum conservation, authors build up the full scope real time simulation mathematical model of super critical steam generator. The dynamic experiments of feed water disturbance, power disturbance, Helium flux disturbance are made on the basis of the model. The experiments show that the model of super critical steam generator has excellent dynamic characteristics.