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The monsoon fringe region in North China (NC) is also an ecologically fragile zone. Improving our comprehension of the paleoclimate variations and their driving mechanisms in this region has great significance for environmental protection and agricultural economic development. In order to provide more reliable data for future climate forecasting and reduce the effects of climatic disasters in NC, we established a 328-year stable oxygen isotope (δ¹⁸O) chronology based on four <i>Pinus tabulaeformis</i> Carr. from Mt. Hasi, Gansu Province, and found that the tree-ring δ¹⁸O inherited the signals of summer (July–August) monsoonal precipitation δ¹⁸O (δ¹⁸O_P). Correlation function analysis indicated that the tree-ring δ¹⁸O series responded significantly to the observed local relative humidity from July to August (RH_JA) with <i>r</i> = −0.65 (<i>n</i> = 55, <i>p</i> < 0.001). Based on the clear physiological mechanism, we reconstructed the RH_JA variations from 1685 to 2012 using a transfer function. Our reconstruction was very stable and had strong spatial representativeness, it was significantly positively correlated with Asian summer monsoon (ASM) indices, indicating that our reconstruction reflected the variations of ASM to a large extent. The RH_JA series successfully captured the weakening of the ASM since the 1930s. There was a close connection between the reconstructed sequence and the East Pacific sea surface temperature (SST). Further analyses revealed that El Niño–Southern Oscillation (ENSO) and Pacific Decadal Oscillation (PDO) may play important roles in the summer monsoon precipitation in NC.