Abstract: High starch maize hybrid Si-dan 19 was used to investigate the effect of population density （45000,60000,75000, 90000plants/hm2） on assimilates accumulation and transformation at grain filling stage in cold region.The results showed that： Increasing of starch contents are similar to decreasing of soluble sugar contents with the increasing of plant density,which indicated that soluble sugar transformed starch. Free amino acid contents and slouble protein contents are unacted on plant density at grain filling stage.At early grain filling stage fat contents decreased,but at mid-late grain filling stage fat contents increased with the increasing of plant density.