Abstract: The zero-output condition of low pressure cylinder is the main method for modifying the flexibility of the current heating unit. According to the current deep peak shaving compensation policy of the power grid, it is a practical problem to calculate the benefit of the unit under different working conditions. A fast algorithm for part-load conditions based on the heat balance law is studied, which can quickly calculate the operating status of the unit after condition has changed while ensuring the basic calculation accuracy. Taking a 350 MW supercritical heating unit as an example, we calculated parameters including the power generation, heating power, and heat consumption under 100% TMHCR（maximum heating） and 40% THA operating conditions before and after zero-output condition of low pressure cylinder. Based on this, revenue from power generation and heating was calculated. The results show that low-pressure cylinder zero output operation under 100% TMHCR working condition can reduce power generation load by about 8% and increase heating load by about 27%, while 40% THA working condition can reduce power generation load to 25%Pe, showing significant economic benefits.