Using the field test method, through measuring annual growth and soil water content of dwarf Fuji(self-rooted anvil M_9-T337) in the field of Northern Shaanxi, long-term sequence meteorological data was collected and the potential evapotranspiration (ETc) of young mountain apple trees was calculated according to the Penman formula, and the result was compared with the annual rainfall of the year, then the water requirement of the second year saplings in the experimental area was obtained. The results showed that under the rainfall conditions in 2017, the sum of rainfall in the experimental area was greater than the sum of the water requirement of the saplings, but there was a stage of water shortage before July; further analysis showed that, no matter under the conditions of wet year, general year, general drought year or special drought year, due to the uneven distribution of rainfall time in Northern Shaanxi, although the annual rainfall was enough to meet the water demand for growth of fruit tree, there wasa serious water deficit before July. There even was water deficit before October in the special dry years. Four to five times of supplementary irrigations were needed to carry out before the germination of the fruit trees (March), the long-term growth of the branches (April), the flower bud differentiation period (May and June)and the flowering period (July), and the irrigation quota was determined according to the previous rainfall and the current soil water content. The irrigation amount range was 60%~80% of the orchard field water holding capacity.