Once-through steam generator systems with supercritical fluids are applied to achieve large capacity and high efficiency in power generation systems. A supercritical fluid is considered as a single-phase substance, but properties of the fluid sharply change near the pseudo point, where specific heat of the fluid reaches its maximum at the corresponding pressure. Therefore, properties calculation and correlations of the heat transfer coefficient and pressure drop have to be evaluated carefully to predict the dynamic behavior. In this study, a 1-dimensional discretized dynamic model was developed for the dynamic simulation of the once-through boiler with supercritical fluids. The mass, momentum and the energy balance were solved in each cell. Validation of the model was checked against the experimental data for the single tube, and the dynamic behavior was simulated for the case of the simple step disturbance. After checking the single tube, this model was applied to the water-steam flow system of the commercial 500 MWe once-through boiler. The steady state values were compared with the design data. To simulate the dynamic behavior of the once-through boiler system, the feed water flow rate and the heat duty of the evaporator were changed as the disturbance. Finally, the operation simulation of the water-steam flow of the once-through boiler was performed for the case of dynamic step-down from the supercritical pressure to the subcritical pressure.