Abstract: The steam extraction of the middle and low pressure connecting pipes of steam turbine is a very common production mode of cogeneration. The rationality of extraction parameters plays an essential role in unit operation economy. In this study, a 330 MW cogeneration unit was selected as the research object. The Ebsilon software was used to model and simulate the operating conditions of the unit under different power loads, extraction pressures and extraction flows respectively. Heat consumption characteristics, pressure regulation critical characteristics and energy consumption critical characteristics were analyzed. The simulation results were compared with the experimental results. The results show that, there is a critical curve of energy consumption when extracting steam at the middle and low pressure connecting pipes of steam turbine. When the unit is running in the left area of the curve, the unit heat consumption under heating condition is higher than that under pure condensing condition. And heat supply will increase the overall coal consumption level of the unit, leading to coal shortage in power plant production. When the unit is running in the right area of the curve, the unit heat consumption under heating condition is less than that under pure condensing condition, and heat supply can improve the thermal economy of the unit. Therefore, some of the cascade utilization transformation will increase the loss of throttling and pressure regulation, thus leading to an increase in energy consumption. It is suggested that after the heating and heating transformation of the thermal power unit, the heat supply steam quantity and steam source parameters between units should be reasonably planned by test and calculation, which has a very important impact on the thermal economy of the unit.