As important constituents of a water molecule, the stable hydrogen and oxygen isotopes are ideal conservative tracers for water source, and thus, have been widely used in water cycle research. Precipitation is the main input of water resources; therefore, analysis of its hydrogen and oxygen isotopic compositions can provide basic information for water cycle. Until now, some research has been carried out for the arid and semi-arid regions on the Loess Plateau; however, few studies have focused on the semi-humid region. The Changwu Loess Tableland, which is located in the southern part of the central Loess Plateau and is characterized by a warm temperate, semi-humid, continental monsoon climate (annual mean precipitation is 578 mm), has been studied. In this study, the hydrogen and oxygen isotopic compositions (²H, ¹⁸O and ¹⁷O) of precipitation and some climatic factors (temperature, precipitation amount, humidity) of four years (2005, 2010, 2012-2013) have been determined fort the investigation of the vapor source of precipitation. Both seasonal and inter-annual variations were detected from the isotopic compositions of the precipitation. The isotopic compositions were more depleted from July to November, and the δ¹⁸O and δD varied from -12.42‰ to -9.96‰ and from -88.62‰ to -65.18‰, respectively; however, they were more enriched from February to June, and the ranges of δ¹⁸O and δD were -7.89‰ to -5.08‰ and -50.08‰ to -27.90‰, respectively. The most abundant depleted isotopic compositions were observed in July, which was considered to be the effect of marine air mass and precipitation amount. δD and δ¹⁸O had a significant relationship with the precipitation amount from June to September, and while they correlated well with air temperatures from October to May. The inter-annual variations in isotopic compositions of precipitation were mainly due to the climate variability; for example, the temperatures in the summer of 2005 were the highest since 1951, and it would, thus, yield different seasonal patterns of isotopic compositions as compared to the other three years. Meanwhile, the isotopic compositions of precipitation in the Changwu Loess Tableland were different from those of the arid regions and similar to some of the humid monsoon regions. According to the analysis of isotopic compositions, meteoric water line, D-excess, and ¹⁷O excess, the marine air mass was the dominant vapor source for precipitation from June to September, especially from July to September, while precipitation from October to May were significantly influenced by regional evaporation and continental air mass. Furthermore, the weighted average D-excess of most months was higher than the global average (10‰), which implied that the isotopic compositions were influenced by local evaporation throughout the year. In rainy season, a few precipitation events were directly associated with marine air mass, while the other events occurred only when the relative humidity exceeded 90% due to the reallocation of marine air mass. At least 30% of the precipitation events have experienced serious secondary evaporation. Analysis of the vapor source by ¹⁷O is still at the preliminary stage and few researches have been done in China; therefore, studying ¹⁷O concentrations can provide additional useful information to interpret the water cycle in future research.