Abstract: Flowers were labeled with cards during blossom of Astragalus membranaceus var. mongholicus plants cultivated in Weiyuan county, Gansu. The dynamic filling trends in the plant seeds were studied 10 d after blossom to provide a theoretical and technical basis for standards of seed production. The 100-grain fresh weight increased and reached its highest value on the 31st day following blossom, and then rapidly decreased to a level near its dry weight. Changes of dry matter fitted a Logistic equation with a rapid increase during 19－33 d after blossom and finished at the end of 50 d. Grain filling rate showed a “fast-slow-fast-slow” pattern. Grains dehydrated increasingly faster with prolonged filling duration and water content decreased to the end. The time of fastest decrease in water content indicated the finish of the maximum grain filling. All the above showed that the stability of dry weight and water content of seeds were important indicators for seed maturation. The best harvest time should be during the 50－56 d following blossom (at the end of July or beginning of August) during which time the water content of seeds should be lower than 3.0%, the seed pods are not bursting apart and they have no abscission-layer in their stalk. The seeds should be harvested in timely installments.