A chemical method was developed for determination simultaneously of nitrogen and oxygen isotope ratios of nitrate in water samples. This method uses copper-plated cadmium granules to reduce nitrate into nitrite in a weakly alkaline environment and sodium azide to transform nitrite into nitrous oxide in a weakly acid buffer. Then the nitrous oxide is purged from the water sample and trapped cryogenically using an automated system with subsequent release into a gas chromatography column. The isolated nitrous oxide is then analyzed with a continuous flow isotope ratio mass spectrometer for N and O isotope ratios, which is then converted into those of nitrate using an equation. Meanwhile, the experimental conditions of chemical conversion have been optimized by some further experiments. It is found that pH of the solution, reaction time and concentration of nitrate all had some effects on processes of the reducing reaction; When pH of the solution was 8.0, N₂O generation rate reached the highest; When duration of the reaction exceeded 80 min, the reaction was nearly 100% completed; And temperature of the reaction system did not have much obvious effect on completeness of the reaction, but the time of reaching equilibrium. On such a basis, optimal conditions for the reducing reaction are set as: 8.0 for pH, 80min for duration of the reaction and 25℃ for temperature of the reaction system. To prepare a series of standard solutions to form a gradient in nitrogen and oxygen isotopic abundance, two standard sample solutions, USGS34 and USGS32，the same in concentration 20μmol L^-1, were blended in different volume ratios. The standard solutions were used in transformation experiments and eventually a conversion working curve was obtained. Besides, a number of validation experiments were carried out using USGS international standard samples and laboratory standard samples to verify precision and accuracy of the chemical conversion method. The conversion process has only two steps and needs only 4.5μg of NO3--N, while its accuracy in measuring δ¹⁵N and δ¹⁸O reached 0.31‰ and 0.55‰, respectively. The difference between the measured value and the reference value was less than 1σ. So the precision and accuracy of this method are good enough to meet the requirements of the research. Water samples from different sources, including surface water, rainfall and extracted soil solution, were analyzed, using this method. It was found that in rainfall water, δ¹⁵NAir of nitrate was +10.5‰ on average, ranging from +4.5‰ to +18.4‰ and δ¹⁸OSMOW of nitrate was +76.9‰ on average, ranging from +54.8‰ to +84.3‰; in surface water, δ¹⁵NAir of nitrate was +8.9‰, on average, ranging from -1.4‰ to +14.4‰ and δ¹⁸OSMOW of nitrate was +4.8‰ on average, ranging from -2.7‰ to +28.2‰; and in soil solution, δ¹⁵NAir of nitrate was +3.5‰ on average, ranging from -4.3‰ to +8.6‰ and δ¹⁸OSMOW of nitrate was +6.0‰ on average, ranging from -0.1‰ to +13.4‰, which were in good agreement with those in previous researches.