Based on the climatic data and plant phenology observation data of Xi'an from 1979 to 2018, we analyzed the growth trends of four species of deciduous tree during the full leaf development period as well as the full leaf discoloration period using methods of segmented regression and trend tendency rate. We studied the relationship between plant phenology variation and climate change through correlation and Partial Least Squares (PLS) regression analysis. The analyses generated four primary results. First, from 1979 to 2018, the spring phenological periods of four kinds of trees were advanced while the autumn phenological periods were postponed. The length of the growing seasons was extended as a whole. Second, the spring and autumn phenological periods of Xi'an area had abrupt changes in 1982. After the abrupt change, plant phenological characteristics changed dramatically compared with the overall trend, showing that the spring advance rate and autumn postponement rate were faster. The spring phenological period has advanced at a rate of 3.8 days per decade, while the autumn phenophase has delayed at a rate of 4.7 days per decade. Third, the significantly negative correlation existed between the blooming leaf period and the air temperature. The leaf discoloration period positively correlated with the air temperature. However, the influence of precipitation on plant phenology was not significant. The spring phenology and sunshine hours showed very significantly negative correlation. Insignificant positive correlation was observed between the autumn phenology period and sunshine hours. Finally, both the spring and the autumn phenophase were jointly affected by the climatic factors, with air temperature as the predominant factor. The order of the explanatory power of the influence factors on the phenological period is temperature > sunshine hours > precipitation. The rise of the temperature during the same period plays the dominant role in advancing the tree leaf blooming period and the postponement of the leaf color change period.