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Compared to tap water delta H-2 and delta O-18, tap water O-17-excess preserves additional information about source water dynamics. In this study, we provide the first report of O-17-excess variations of tap water across China (652 samples). Annual O-17-excess of tap waters at the national scale did not show obvious spatial pattern, and was almost unaffected by local environmental factors except in the Qinghai-Tibet Plateau region with a strong latitudinal trend. The mean O-17-excess values in different seasons were not significantly different. The isotopic compositions of most of the tap waters at the annual and seasonal scale were likely influenced by the equilibrium fractionation effect (delta'O-18-delta'O-17 slope ranged from 0.5277 to 0.5301), except for the northwest region in the summer (slope = 0.5264) influenced by kinetic fractionation associated with re-evaporation effect. Based on the information of tap water source distribution, site aridity index and the known precipitation delta O-18 values, a subset of the tap water can be considered as precipitation proxy. Different from the obvious spatial characteristics of precipitation delta O-18, precipitation O-17-excess did not show a clear spatial pattern. But it revealed much detailed precipitation formation mechanisms related to different climate regions and geographical conditions. The lower O-17-excess values of the precipitation-sourced tap waters were caused by kinetic fractionation associated with supersaturation process in snow or glacier formation and re-evaporation effect in some arid regions. The higher O-17-excess values of the precipitation-sourced tap waters in the inland were caused by continental moisture recycling, while likely caused by multiple factors in the southeast coastal region including short transport from ocean source and the humid local environment. Overall, this study provides a unique tap water O-17-excess dataset across China, and probes the precipitation formation mechanisms using tap waters. (C) 2019 Elsevier Ltd. All rights reserved.