Abstract: A three year old stand of Elymus sibiricus, was studied with the aim of determining the change in reproductive stem biomass, water content, biomass and energy distribution among floral components during the seed head development, in order to better understand the energetics of seed head development. During flowering various measurements were made, including photosynthetic rate of reproductive culms, biomass and energy distribution (including proportional allocation to stem, leaf, peduncle, and flower), 1000-seed weight, hormone concentration. Measurements were repeated at stem elongation, anthesis, seed filling, the milk stage, the dough stage and seed maturity. Key results were: 1) The total biomass of reproductive stems significantly increased from anthesis to seed filling to the milk stage (P<0.05). Seed head biomass peaked when plants were in the dough stage. The biomass proportion of vegetative organs gradually decreased with seed head development. Leaf comprised 18.39% of total biomass at anthesis, and that dropped to 9.76% at seed maturity. The proportion of stem biomass reached a maximum at the milk stage, then significantly decreased. The biomass of reproductive organs, especially the flowers, significantly increased from the milk stage to the dough stage. However, the biomass of peduncle was relatively static. 2) The photosynthetic rate of reproductive stems changed little from anthesis to the milk stage, but dropped significantly after the milk stage (P<0.05). The total energy content of reproductive stems reached its maximum at the dough stage, but two peaks were observed; one between anthesis and seed filling, and one from seed filling to the milk stage (P<0.05). Energy distribution differed significantly between seed head components (P<0.01), and ranked in order: panicle>leaf>stem>peduncle. 3) The energy content of stems and leaves declined during seed development. The energy content of leaves showed two obvious troughs, with a minimum of 13.69%, from seed filling stage to the dough stage. The energy content of stems was declined significantly from milk the stage to seed maturity. 4) During seed development, seed fresh weight significantly increased twofold, and seed dry weight significantly increased fivefold (P<0.05). Seed yield (units are needed) significantly increased from the milk stage to the dough stage, then significantly decreased after the dough stage (P<0.05). 5) The gibberellic acid concentration fell by 50.25% from seed filling to seed maturity, while abscisic acid concentration increased by 91.37% from the dough stage to the seed filling stage. Therefore, seed production fields should be managed so as to maintain the integrity and health of leaves until the milk stage of seed development, to provide energy for seed development. Seed should be harvested at the end of the dough stage.