Based on the technology of stable isotopes of δD and δ18O, we investigated the seasonal variations of stable isotope of hydrogen and oxygen in rainfall, irrigation water and groundwater from April, 2012 to March, 2014 in an agricultural watershed. Meanwhile, groundwater sources and the seasonality of groundwater recharge were determined by using δD-δ18O plot and dexcess mass balance method respectively. These results showed that the local meteoric water line was δD = 8.49δ18O + 16.7 (n = 110, R2 = 0.98). Large ranges of δD (–113.3‰ – 7.5‰) and δ18O (–14.9‰ – –0.9‰) were found in precipitation, while δD and δ18O values in groundwater exhibited relative steady trend, ranging from –45.3‰ to –40.4‰ and –7.29‰ to –6.44‰ respectively. The variations of δD (–66.2‰ – –28.3‰) and δ18O (–9.17‰ – –5.00‰) in irrigation water were less than precipitation but greater than groundwater. In δD-δ18O plot, groundwater samples were virtually identical to the local meteoric water line, indicating that groundwater was mainly recharged by atmospheric precipitation. Deuterium excess values of precipitation ranged from 1.3‰ – 23.7‰, with the average of 10.8‰ in summer half year and 17.1‰ in winter half year respectively. Compared to precipitation, the dexcess of groundwater was relatively stable (8.75‰ – 14.9‰), with the average of 11.9‰. Using dexcess mass balance method, 83% groundwater was recharged in summer half year and only 17% in winter half year, representing a distinct seasonal variation of groundwater recharge and the importance of precipitation in summer half year to groundwater recharge.