Regenerative system is an important part of steam turbine system in power plant. The regenerative heaters models in off-design conditions are very complicated due to the heaters phase-change problem. A dimensionless model for regenerative heater is built up according to actual measurements which reflects its operation status in off-design conditions. This model is applied to the first low-pressure heater of a 660 MW ultra-supercritical steam turbine unit. Results show that the deviation of model calculation results and actual data are very small. The modeling method is simple and credible and can be used as tools for practical troubleshooting in power plant.