The East Asian summer monsoon (EASM) is a key atmospheric circulation system that regulates the hydroclimatic patterns and ecological processes of the Loess Plateau. Its onset date has important implications for the seasonal distribution of regional precipitation, water resource allocation, and land-atmosphere interactions. In this study, we developed a quantitative framework for identifying monsoon onset based on the daily time series of water vapor 518O. Using observed water vapor isotope records from Lanzhou and Xi'an across the Loess Plateau, we determined a robust onset signal characterized by a maximum in 518O followed by sustained isotopic depletion. After applying a 10-day Fourier low-pass smoothing, objective criteria with a cumulative decrease of at least 2 parts per thousand were established to identify the monsoon onset date. This framework was further applied to longterm daily isotope simulations of IsoGSM4 to reconstruct the interannual variability of EASM onset over the Loess Plateau. The results show that the summer monsoon typically reaches the Loess Plateau in June and exhibits a stable southeast-to-northwest propagation pattern. On average, monsoon onset occurs in Xi'an in early June, whereas Lanzhou lags by approximately 15-20 days. The inter-annual variability of monsoon onset date is consistent with recent evidence for a northward shift of the East Asian rain belt. This isotope-based framework provides a robust quantitative indicator for identifying EASM onset across the Loess Plateau, and helps in understanding the long-term hydrological changes in the monsoon marginal regions.