Based on the water balance equation, a field experiment was carried out to observe the dynamic characteristic of volumetric soil water content during growing season of 2012 and to explore the impact of irrigation on the oasis water budget. The results showed that in the Zhangye oasis, the volumetric soil water content at the depth of 40–60 cm was higher than that at the depth of 0–20 cm during days without precipitation and irrigation. Heavy precipitation has an obvious impact on the volumetric soil water content at the depth of 0–20 cm while irrigation has an obvious impact at the depth of 0–100 cm. The daily evapotranspiration (ET) was 2.83 mm/d during days without precipitation and irrigation. The ET volume of the third day after precipitation increased by 16% compared to the ET volume before precipitation. The ET rate of the second day after irrigation was observed with an increase of 88% compared to the ET rate before irrigation, indicating that the impact of irrigation on ET was more significant than that of precipitation. The primary incomings of the water budget in the oasis was irrigation, which accounted for 89.7% of the total incomings. The main outgoings of the water budget in the oasis was deep percolation (DP), which accounted for 81% of the total outgoings. The irrigation water requirement was only 213 mm during the growing season of 2012, but the real irrigation volume exceeded the irrigation water requirement by 474 mm. Excessive deep percolation and serious water wastage was obvious during growing season due to the heavy irrigation.