Abstract: With the rapid growth of the installed capacity of new energy, the contradiction between heating and power supply of cogeneration units is becoming increasingly prominent, and the flexible removal of low pressure cylinder transformation has been widely used in cogeneration units, which can effectively improve the heating capacity and peak load capacity of the unit. A 330 MW direct air-cooled heating unit is taken as the research object, and the analysis and calculation model of the unit's variable working conditions before and after transformation of the low pressure cylinder based on ultra-low back pressure operation is built by the Ebsilion software. The safety of low pressure cylinder blade under low volume flow condition is studied experimentally. Based on the working diagram, the characteristics of electric heating load, peak load capacity and standard coal consumption of power generation before and after the transformation are analyzed. The results show that: after the removal of the low-pressure cylinder based on ultra-low back pressure operation, the maximum heating capacity of the unit is increased by 80 MW on average; the peak load capacity is increased by 52 MW on average, and the minimum generation standard is reduced by 42 g/kWh on average. The low pressure cylinder cutting technology based on ultra-low back pressure operation effectively solves the problems such as water erosion, flutter and blast of the low pressure cylinder blade, winter antifreeze pressure of air-cooled heating units in the flexible transformation work, and eliminates the hidden danger of the unit. At the same time, the technology has the characteristics of flexible adjustment of low-voltage cylinder output, which not only improves the thermo-electrolytic decoupling ability, but also provides conditions for the unit to participate in the auxiliary service of the power grid. This paper provides a quantitative basis and theoretical support for the application of flexible cutting low-pressure cylinder reconstruction technology based on ultra-low back pressure operation.