Abstract: This study uses the trivalent titanium reduction method to convert nitrate ions in standard samples of seawater, lake water, and rainwater into N₂O to test their nitrogen and oxygen isotope values, and optimize experimental conditions (NO^-₃ concentration, reagent amount, reaction time, and temperature, etc.). Results show that when 4 cm³ nitrate water sample with concentration approximately 20 μmol/dm³, added with 100 mm³ TiCl₃ solution 150 g/dm³ and reacted at 25 ℃ for 24 hours, the average conversion rate of NO^-₃ reduction to N₂O is 86.3% (n=5). The slopes of the calibration curves for 5 nitrate nitrogen and oxygen isotope standards are 0.947 and 0.617, respectively, and the correlation coefficients R² are 0.999 and 0.994, respectively. The detection limit of the method is 2.5 μmol/dm³ for NO^-₃, of which the standard deviation of δ¹⁵N is less than 0.7‰ (n=5), the standard deviation of δ¹⁸O_VSMOW value is less than 2.5‰ (n=5), standard deviation of δ¹⁵N is less than 0.5‰ (n=5), and standard deviation of δ¹⁸O_VSMOW value is less than 1 ‰ (n=5). Thus, three types of water samples measured by the method have resulted in standard deviation of 0.3 ‰, 0.6‰, and 0.5‰ for δ¹⁵N, respectively and standard deviations of 2.1‰, 2.0‰ and 1.6‰ for δ¹⁸O_VSMOW, respectively. The Ti³⁺ reduction method is simple and efficient for the determination of nitrogen and oxygen isotope of NO^-₃ in water, but the oxygen value needs to be corrected systematically.