Desert steppe is distributed in the semiarid and arid areas and is extremely sensitive to climate change. However, limited field observations and lack of community surveys have resulted in insufficient research on the vegetation phenology and productivity of the desert steppe. Based on the normalized difference vegetation index (NDVI) data from the MODIS dataset during 2000-2017, we used a relative threshold method to extract the phenological parameters in the desert steppe of Inner Mongolia, including the start of growing season (SOS), the end of growing season (EOS), and the length of growing season (LOS). We then estimated the spatiotemporal changes in net primary productivity (NPP) of the desert steppe vegetation by the Carnegie-Ames-Stanford Approach (CASA) model. Finally, we analyzed the relationship between desert steppe productivity, phenophases, and climate variables. Our results show that: 1) during the study period, SOS advanced significantly at a rate of 0.88 d/a (P<0.05), while EOS advanced at a rate of 0.13 d/a (non-significant). The average LOS lengthened significantly by 0.76 d/a (P<0.05). The SOS was correlated negatively with mean temperature from February to April in 81.53% pixels (8.21% was significant) and negatively correlated with April precipitation in 60.80% pixels (6.12% was significant). The EOS showed a negative relationship with mean temperature in September in 65.16% pixels (5.03% was significant) but positively correlated with precipitation from July to September in 78.61% pixels (10.12% was significant). 2) The average annual NPP from 2000 to 2017 was 104.71gC/(m ²·a) in the study area, showing regional differences with an obviously decreasing trend from east to west. Net primary productivity in spring, summer, and the growing season increased insignificantly, while NPP in autumn showed an insignificant decreasing trend. The increase of precipitation in the growing season is beneficial to the accumulation of ecosystem NPP. 3) Advance of SOS was conducive to the accumulation of spring NPP, and the delay of EOS promoted the accumulation of autumn NPP. There was a significant correlation between the LOS and NPP during the growing season (P<0.05). This study revealed the impacts of climate change on vegetation phenology and productivity of the desert steppe in Inner Mongolia, which is significant for ecosystem management and ecological construction of the region.