During startup, no-load and low-load of the steam turbine, the steam flow is less, which is not sufficient to take away the heat generated by friction of air blowing inside the low-pressure cylinder, thus causing the exhaust air temperature to rise and the exhaust casing to rise too. The excessive high temperature of the exhaust casing will cause the casing to deform greatly and damage the consistency of center line of the dynamic and static parts of the steam turbine, and will cause the vibration or other accident of the units in severe case. Therefore the exhaust casing of the steam turbine is provided with low-load and water spraying device.