To further understand the hydrogen and oxygen stable isotope composition differences between fog water and precipitation in different terrestrial systems and major influencing factors, the global-scale δ 2H and δ 18O research data for fog water and precipitation were collected and the significance of their differences was statistically analyzed. Combing the calculation of fog water’s lc-excess values, the isotopic difference rules between fog water and precipitation were classified, and corresponding causes and relations to fog types were further discussed and revealed. Results suggest that generally the fog water is significantly more enriched in heavier isotopes than the precipitation, with δ 2H and δ 18O differences of 10‰ to 60‰ and 1.2‰ to 5.5‰, respectively. The differences between fog and precipitation isotopic composition can be classified into three categories based on lc-excess values of fog water: the lc-excess value is approximately 0 and the fog water is within the 95% confidence interval of LMWL; the lc-excess value is greater than 0 and the fog water is distributed above the LMWL; the lc-excess value is less than 0 and the fog water is distributed below the LMWL. The water vapor source, condensation temperature and environmental temperature are three major factors that influence the isotopic difference between fog water and precipitation, with different categories correspondingly dominated by different factors. Orographic fog, radiation fog, and advection fog have different water vapor conditions and condensation processes, leading to different patterns of isotopic differ ences in fog and precipitation. There is no significant difference in the isotopic composition between terrain fog and precipitation, while there is a significant difference in the isotopic composition between radiation fog and precipitation, with significant enrichment of heavy isotopes in fog water. The overlap area of isotopic composition between advection fog and precipitation is small, and heavy isotopes in fog water are obviously more enriched.