Because of the problem once happened in the process of opening the valve, a serious accident occurred in a power station when the steam turbine was being in operation. All of stages were ruptured in the low pressure cylinder. The rupture and crack of diaphragm has been studied strictly. Some experts drew a conclusion that the diaphragms are broken by great pressure difference. But this conclusion lacks theoretic support. In the paper, the oscillating flow field in low pressure cylinder is calculated using the theorem of oscillating fluid mechanics. The conclusion drawn by experts is verified by the results calculated.