Abstract: For promoting renewable energy integration in the electricity market, the 2016-2020 Development Plan of Flexibility Upgrading for the Fossil-fired Generation Units was stimulated by the China National Energy Administration, in which ambitious targets are set out for increasing the peak load adjustment capability of the existing fossil-fired units. It’s expected that peak load adjustment capability of 20% rated load be increased and minimum output of 40%-50% rated load be reached by the flexibility upgrading of the cogeneration units. This set a stringent upgrading target for the existing cogeneration units. Focusing on solving the interference of the heating supply and power generation under relatively high heating load conditions, heating-power decoupling of the cogeneration units can lower the power output of the generation units to a great extent while meeting the external heating load requirement. The solution of heating steam system upgrading, has the advantage of minimum equipment modification and lower investment. The objectives of the article including: (1). to study the peak load adjustment capability of typical 300MW subcritical cogeneration unit under various heating steam modification schemes using model-based thermal system simulation method; (2). to analyze the economic performance of cogeneration unit under various heating steam modification schemes. The heating steam system modification schemes provided for the heat-power decoupling, take the high pressure steam sources as the supplement for normal heating steam extractions under high heating load conditions. Because of the high enthalpy of the new introduced steam source, the required heating steam flow can be reduced and the resulting remaining steam inlet flow for the LP casing can be increased, which improves the unit peak load adjustment capability under high heating load conditions. Model-based simulation code is used for the performance simulation of the 300MW subcritical cogeneration unit manufactured by STC. Three possible heating steam modification schemes are considered in the modeling building-up process, and the critical operation parameters and limits are explained for the modification schemes. The high pressure supplement steam is taken from the IP turbine inlet hot reheat steam and HP turbine exhaust cold reheat steam and No.4 steam extraction respectively for Scheme No.1, No.2 and No.3. It is shown that with the increase of the proportion of the high pressure steam, the minimum load that could be reached is decreased while maintaining the 97% maximum heating load. From the point view of the peak load adjustment improvement, scheme No.1 has apparent and significant effect and scheme No.2 has average and moderate effect while scheme No.3 has the worst effect. In scheme No.1, the turbine inlet steam flow can be decreased to 827t/h and the power output can be lower to 143.92MW under 97% maximum heating load while maintaining the minimum LP turbine cooling steam flow requirement. A heat rate of 5808.56kJ/kW.h is expected under this condition. Although the performance is deteriorating compared with the original scheme, the performance is comparatively not very far from the unit performance under 75% maximum heating load of original scheme. Therefore scheme No.1 is the optimum modification scheme for heat-power decoupling of the existing cogeneration unit. For other modification schemes as No.2 and No.3, although the performance is not deteriorating greatly compared with the original scheme, the results of the peak load adjustment improvement are relatively poor. Therefore, these schemes are not recommended in the heat-power decoupling measures.