The developing trend that digital instrument and control( I&C ) system will supplant traditional analog I&C system in nuclear power plant is emphasized. This paper introduces mathematical model of steam generator of full scope simulator for nuclear power plant. The independent real-time simulation system

which forms the interac- tive closed loop together with the steam generator control system

can be applied to provide a simulation target for upgrades of instrument & control system and the research of control schemes. At the same time

a simulation method for this purpose is presented in this paper. In this method all of the hardware and software are composed of real dis- tributed control system except the model of controlled object. This will not only create favorable conditions for com- missioning on site in the future

but also give a theoretical analysis for upgrades of digital I&C system in nuclear power plant.