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Terrestrial snails are sensitive to climate changes and their shell's oxygen isotope composition (δ18O shell) is widely used in studies of paleoclimate reconstructions. However, the interpretation of δ18O shell remains complex due to the combined effects from different factors such as precipitation δ18O (δ18O p), relative humidity (RH), and temperature. Furthermore, no systematic studies have addressed the transformation of oxygen isotope signals from rainfall to the aragonite shell via snail body water. Here we present the results of Cathaica fasciola body water δ18O (δ18O BW) and its content, which were sampled every two days throughout the growing season in 2021 in Xi'an. Meanwhile, the soil water δ18O (δ18O SW), δ18O p , rainfall amount, RH and temperature were also simultaneously monitored, and two high-resolution (at ∼0.3 mm interval) δ18O shell series were obtained from two live snails collected in September and October 2021, respectively. These data reveal that the δ18O BW mimic the variations of δ18O p , and they are positively correlated (r2 = 0.84) for precipitation events and the correlation coefficient only increases slightly (r2 = 0.88) when RH is included in the multivariate analysis. During non-precipitation intervals, there is an obvious negative correlation between δ18O BW and RH. Moreover, the variations of two δ18O shell series are broadly consistent with the theoretically calculated δ18O shell from δ18O BW /δ18O p at seasonal scale. Collectively, this study demonstrates that δ18O p is the first-order control on the seasonal variations of δ18O BW and δ18O shell , while RH play its role during non-precipitation periods. Importantly, this study lays good foundation for reconstructing terrestrial seasonal distribution of precipitation and extreme rainfall events using fossil shells in the geological past. • Precipitation δ18O paced the seasonal variations of snail body water δ18O. • Relative humidity plays its role on body water δ18O during non-rainfall periods. • High-resolution snail shell δ18O recorded seasonal precipitation δ18O signals. [ABSTRACT FROM AUTHOR]