Objectives: Water is an important component of fluid system carrying information on the nature and evolution of fluid. Water is usually extracted from fluid inclusions by mechanical vacuum crushing technique and thermal decrepitation. An online extraction device was invented to reduce water adsorption of surface increasing by vacuum crushing and isotope exchange reaction by thermal decrepitation and measure δD in water from calcite mineral inclusions.Methods: The online extraction device was connected with thermal decrepitation elemental analyzer plus isotope ratio mass spectrometry in pure helium. The key technical parameters of hydrogen isotope composition in Water from Fluid Inclusion of Calcite mineral were confirmed by carrying out a series of conditional experiments.Results: After a series of conditional experiments, the parameters are optimized with the 420℃ reactor temperature, 5 min cracking reaction time, 0.1～0.3 μL water content of mineral inclusions. The precision of δD in water from calcite mineral inclusions is better than ±1.5‰ (1σ).Conclusions: The method can be used to extract water from calcite mineral fluid inclusion online and measure the hydrogen isotope composition online, which simplifies the operation process and improves the efficiency.