Modern investigations have shown that oxygen and carbon isotopes of land snail shells are useful indicators of climate and vegetation in monsoonal region. However, stable isotope study on snail fossil shells in strata has been seldom done, and the reliability of those indicators needs further verification. Moreover, intra-shell stable isotope analysis of individual snail is rather scarce, and seasonal variation of the glacial-interglacial monsoonal climate remains unclear. In this context, we performed δ18O and δ13C analyses on fossil shells of cold-aridiphilous Cathaica pulveratrix and sub-humidiphilous Metodontia yantaiensis from the loess section over the last two glacial cycles at Beiyao site in southern Chinese Loess Plateau. The δ18O of fossil shells reflected monsoonal rainfall amount and more rainfall during MIS3 and MIS7. Meanwhile, the δ13C of fossil shells indicated relative abundance of C3/C4 plants and more C4 biomass during MIS3 and MIS7. The δ18O and δ13C of the two species from the same horizon are significantly different, reflecting differences in their growing season and/or physiological habits. Intra-shell variations of stable isotopes showed that climatic seasonality was relatively strong during the glacial periods whereas seasonality became weakened during the interglacials. Our findings provide an environmental background for explaining past human activities at the Beiyao site. The investigation of stone artifacts showed that ancient human activities were relatively strong during MIS3 and MIS7. During these stages, the warm and humid climate with smaller seasonal contrast was favorable for the regional expansion of human activities.