The relationship between plant phenology and climate factors has been studied mostly based on linear models for a long time. In fact, the response of plant phenology to climate change may be non-linear. Based on long-time series of phenological observation data of temperate typical grassland and temperate steppe desert in Inner Mongolia from 1984 to 2017, the phenological change characteristics of dominant plant species in different grassland types and their non-linear responses to spring precipitation during the past 40 years were analyzed. The results showed that: (1) the controlling factor of Artemisia frigidawilld regreening in temperate typical grassland was water in spring, which behaved as a quadratic function of downward opening. The water deficit caused by warm and dry climate in spring was the main reason for the significantly delayed trend (1.32 d/a) of Artemisia frigidawilld regreening. Only if the precipitation in spring exceeded 60 mm, its regreening might show a transition trend from delay to advance. (2) The controlling factor of Oxytropis aciphylla regreening in temperate steppe desert was temperature. Affected by the significant warming in spring, the regreening of Oxytropis aciphylla showed a significantly advanced trend with the magnitude of 0.63 d/a. The precipitation increase in spring promoted regreening, and there was a quadratic function of upward opening between the plant regreening and spring precipitation. When the spring precipitation exceeded 40 mm, its response to precipitation gradually decreased, which was mainly related to the physiological characteristics of desert plants.