Abstract: To quantitively evaluate the peak shaving economics of coal-fired cogeneration units in the heating season, this paper divides the steam into a condensate and heating flow according to production progress. The essence of peak shaving is revealed by analyzing condensate mass flow and heating mass flow in the peak shaving process. Additional fuel consumption caused by peak shaving, which is used as a quantitative assessment index of peak shaving economics, is quantitively proposed according to the peak shaving depth. The applicable condition is also clarified. The calculation method of the additional fuel consumption by peak shaving, which is used as the basis for calculating the cost of unit electricity and heating, is proposed by analyzing the exergy balance and energy balance of the cycle process of steam. The rationality of the index is verified by a 350 MW cogeneration unit. The result shows that the power output of the heating flow is decided by the heating load. The unit's power output is adjusted by changing the mass flow of condensate flow on the basis of the mass flow of the heating flow. In deep peak shaving mode, the additional fuel consumption by peak shaving is significant when the heating load remains unchanged. The index quantitively characterizes the energy consumption level of the peak shaving process. Combined with the thermoelectric feasible region, the range of applications of different peak shaving modes can be shown intuitively. The index provides a theoretical basis for selecting peak shaving methods and making operation strategies.