The utility model provides a four take out back pressure turbine steam extraction that supplies vapour supplies MED sea water desalination, it includes a main steam turbine, main steam turbine's level four is drawn gas and is connected a back pressure turbine, and this back pressure turbine is connected with a power frequency generator that is used for the electricity generation, back pressure turbine's steam extraction gets into the MED device behind water spray reducing temperature equipment, back pressure turbine connect the electric quantity that the power frequency generator sent out and incorporate the power plant station service electrical system into, back pressure turbine arrange in the MED device regionally, its steam extraction directly reachs the required parameter of MED to can get into the MED device after the temperature spraying water to subtract, it can be avoided taking out and congeal the unit and draw gas the parameter and mismatch with the MED parameter and cause energy loss's problem, improves system efficiency, reduces MED system operation cost.