Abstract: in view of the zero-output transformation of the low-pressure cylinder expansion difference beyond the limit of the problem analysis, it is determined that the main reasons for the expansion difference exceeding the limit are that the second leakage high-temperature steam in the rear seal of high-pressure cylinder returns to the low-pressure cylinder for heating through the seven-draw pipe and the rising temperature of the middle row. Through the temporary measures of closing small inner two leakage door and reducing reheat steam temperature, the expansion difference can be effectively controlled below the high limit to ensure the safe long-period cylinder-cutting operation of the unit. The analysis thinking of air leakage into low-pressure cylinder aggravating blast loss and heating into low-pressure cylinder steam causing expansion difference is correct, which is worth using for reference. The Quantitative analysis of the differential expansion, the time required to reach the peak and the change in the differential expansion caused by the temperature difference are clear and commendable. From the # 1 machine than # 2 machine to expand the difference, increase quickly as the cut-in point of system differences in-depth investigation, screening parameters to determine the true and false key factors of the process is worth learning. In order to solve the problem of increasing the temperature of the middle row, the author Quantitative analysis that the problem of poor heat transfer performance of the heater in the heating network is worthy of reference.