The invention discloses an analysis pretreatment device for stable carbon isotopes in carbonate dissolved in water bodies. The analysis pretreatment device comprises a reagent bottle, a dryer, a valve, an injection pump, a one-way check valve, a high-purity nitrogen bottle, an electromagnetic valve I, an electromagnetic valve II, a reaction flask, a cold well, a vacuum pump, a gas collecting bottle, a magnetic stirrer and an isotope mass spectrometer. The reagent bottle is communicated with external atmosphere via a gas inlet tube; the reagent bottle is communicated with the reaction flask viaa liquid delivery tube; the reaction flask is communicated with the cold well via a gas delivery tube; the cold well is communicated with the vacuum pump, the gas collecting bottle and the isotope mass spectrometer via gas delivery tubes. The analysis pretreatment device has the advantages that CO2 gas in residual air in the analysis pretreatment device can be jointly removed by the aid of vacuumizing and purging processes, and accordingly interference on experimental results can be reduced; samples are stirred by the magnetic stirrer, accordingly, the reaction rate of phosphoric acid and soluble inorganic carbon can be increased on the premise that the gas tightness of the analysis pretreatment device is not destroyed, the reaction time can be shortened, and the analysis efficiency can be improved.