Abstract: Higher analytical precision and lower detection limit are required in geological dating and tracing, due to the wide application of Os. In this study, high-precision determination of pg-ng Os by Negative Ion Thermal Ionization Mass Spectrometry (NTIMS) was established by reagent purification, vessel cleaning and chemical preparation to ensure the ultra-low blank and obtain sensitive and stable signals. Isobaric oxygen interferences are corrected by stepwise spectrum stripping, and mass discrimination effect is corrected by internal standard iteration. The total procedural blank of Os is 0.41 pg, the signal of 1 pg ¹⁹⁰Os is about 85000 cps, and the mass fractionation factor is less than 0.1%. Pg level of Os is measured by ion counter dynamic jumping scan mode and the measurement precision of ¹⁸⁷Os/¹⁸⁸Os is less than 0.2%. ng level of Os using Faraday cup static mode yielded the measurement precision of ¹⁸⁷Os/¹⁸⁸Os less than 0.005%. The measured value, ¹⁸⁷Os/¹⁸⁸Os=0.3361±7 (2RSD, n=9), for national reference material JCBY is identical with the certified value within uncertainty. The analysis precision meets the advanced international laboratory level. This method has the merits of low blank, high sensitivity and high accuracy, and can provide high-precision Os isotope data of geological samples.