The GM (2 ,1) models can reflect the rend and periodic changes of system 111 the meantlme , so it is very useful. But the apprOXlmate calculation m traditional method of building models made the precision of GM (2 ,1) models very low , and greatly obstructed its usmg range. Aimmg at these shortages , this paper presents and proved an lmprovmg method of building GM (2 ,1)models ， and tests and verifies this method by an example.