The length and magnitude of vegetation growing season are important factors affecting the change of vegetation productivity during the growth process. Under the context of global warming, vegetation growing season at the middle and high latitudes of the Northern Hemisphere has prolonged significantly and caused positive feedback on vegetation productivity. However, the change of vegetation growth magnitude and its impact on vegetation productivity are still unclear. Northeast China is located in the mid-latitude temperate zone with high vegetation coverage and various vegetation types. Exploring the change of vegetation growth season length and magnitude and their influence on productivity is meaningful for understanding and coping with ecosystem changes in the study area. Based on the long-term GIMMS NDVI3g remote sensing data (1982-2015), the curvature derivation method was used to extract the key vegetation phenological parameters such as start of season (SOS), end of season (EOS), growth season length (LOS) and growth magnitude (GM). Then the relative importance (RI) method was employed to detect the relative contribution of LOS and GM to vegetation productivity (expressed as mean NDVI value in growing season, MGS) in growing season. The results showed that: (1) The overall vegetation productivity and growth magnitude in the study area showed an increasing trend, while the LOS showed a decreasing trend, which led to the GM becoming the main factor controlling the change trend of productivity (RI = 70%); (2) In different vegetation coverage areas, the impact of growth season length and magnitude on productivity showed significant spatial discrepancy. Vegetation productivity in the western grassland region was most significantly controlled by GM (RI = 93%), followed by coniferous forest and broad-leaved forest (RI = 66%, 62%) and crop area was least affected by GM (RI = 56%). The impact of LOS on vegetation productivity is most significant in croplands (RI = 40%) and affects about 27%-35% in other areas. GM was positively correlated with productivity in all vegetation cover areas, while LOS was negatively correlated with productivity; (3) Both climate factors (precipitation, temperature) and phenological changes affect the main contributing factor GM. In detail, the change of SOS has the most significant effect on the GM in a large spatial range. The main manifestation is that delayed SOS can promote GM. Based on remote sensing technique, this study found that vegetation in Northeast China is generally growing more vigorously, but vegetation growth activities are mainly affected by growth magnitude. This study can provide direct evidence for the study of vegetation phenological changes and productivity response under the background of global change.