Abstract: Based on the mass and energy conservation of once-through boiler’s steam-water system, the water wall was divided by the standard of the IAPWS-IF97, and the dynamic relationship between the valve opening and intermediate point superheat degree of the ultra supercritical units was studied.Subsequently a mathematical model was established between above two variable, with which the goal can be achieved to provide theoretical basis for the improvement scheme of the primary frequency regulation control system. According to a 1000 MW ultra supercritical unit’s operating data and structure parameters, the obtained model was simulated. Results show that the superheat degree decreases with the increase of valve opening at the rated working condition, and the unit will be in an unsafety operating state when the superheat degree exceeds the protection value; in different working condition, the superheat degree variation is smaller and the steam-water system stability is better with the increase of load, but the ability of primary frequency regulation is worse.