Based on core observation and detailed lithofacies analyses in the Cenozoic section, using plenty of analytical data of grain size, a large number of probability cumulative grain size curves in the main terrigenous depositional environments of the northern Qaidam Basin were summarized. The results show that the study area mainly developed alluvial fan deposit in the Lulehe Formaition, braided river in the Lower Xiaganchaigou Formation and braided delta in the Upper Xiaganchaigou Formation. Each sedimentary facies is divided into several subfacies and microfacies again. The characteristic of probability cumulative grain size curves in different sedimentary facies are discussed. Each sedimentary subfacies has its own peculiar probability cumulative grain size curves.