An extraction apparatus for extracting at least one constituent in a substance, comprising: an extractor (11; 21; 31) for placing and mixing said substance and an extraction fluid, wherein said extraction fluid is a subcritical fluid, and said extractor comprises a temperature control unit (113; 213), said temperature control unit is configured to control the temperature of said extractor for making the temperature of said extraction fluid higher than its boiling point and lower than its critical point to keep said extraction fluid as said subcritical fluid and making said at least one constituent in said substance be dissolved in said extraction fluid; an extraction evaporator unit (13; 23) linked up with said extractor (11; 21; 31) for receiving and heating said extraction fluid and said at least one constituent dissolved in said extraction fluid from said extractor to gasify said extraction fluid; an extraction separator linked up with said extraction evaporator unit (13; 23) for receiving and separating said at least one constituent and said gasified extraction fluid from said extraction evaporator unit; and an extraction condenser (15; 25; 35) linked up with said extraction separator and said extractor (11; 21; 31) for receiving and liquefying said gasified extraction fluid from said extraction separator and sending said liquefied extraction fluid to said extractor, wherein said extractor, said extraction evaporator unit, said extraction separator and said extraction condenser are suitable for being operated under constant pressure, wherein said extraction evaporator unit (13; 23) comprises a first evaporator and a second evaporator, said first evaporator being linked up with said extractor to receive said extraction fluid and said at least one constituent from said extractor to perform a first stage heating process on said extraction fluid and said at least one constituent and the second evaporator being linked up with said first evaporator to receive gasified extraction fluid, the at least one constituent and/or extraction fluid from said first evaporator and perform a second stage heating process on said gasified extraction fluid the at least one constituent and/or extraction fluid at a higher temperature than said first stage heating process and linked to said extraction separator to send said gasified extraction fluid and said at least one constituent to said extraction separator. an extractor (11; 21; 31) for placing and mixing said substance and an extraction fluid, wherein said extraction fluid is a subcritical fluid, and said extractor comprises a temperature control unit (113; 213), said temperature control unit is configured to control the temperature of said extractor for making the temperature of said extraction fluid higher than its boiling point and lower than its critical point to keep said extraction fluid as said subcritical fluid and making said at least one constituent in said substance be dissolved in said extraction fluid; an extraction evaporator unit (13; 23) linked up with said extractor (11; 21; 31) for receiving and heating said extraction fluid and said at least one constituent dissolved in said extraction fluid from said extractor to gasify said extraction fluid; an extraction separator linked up with said extraction evaporator unit (13; 23) for receiving and separating said at least one constituent and said gasified extraction fluid from said extraction evaporator unit; and an extraction condenser (15; 25; 35) linked up with said extraction separator and said extractor (11; 21; 31) for receiving and liquefying said gasified extraction fluid from said extraction separator and sending said liquefied extraction fluid to said extractor, wherein said extractor, said extraction evaporator unit, said extraction separator and said extraction condenser are suitable for being operated under constant pressure, wherein said extraction evaporator unit (13; 23) comprises a first evaporator and a second evaporator, said first evaporator being linked up with said extractor to receive said extraction fluid and said at least one constituent from said extractor to perform a first stage heating process on said extraction fluid and said at least one constituent and the second evaporator being linked up with said first evaporator to receive gasified extraction fluid, the at least one constituent and/or extraction fluid from said first evaporator and perform a second stage heating process on said gasified extraction fluid the at least one constituent and/or extraction fluid at a higher temperature than said first stage heating process and linked to said extraction separator to send said gasified extraction fluid and said at least one constituent to said extraction separator.