Abstract: In the present study, photosynthetic parameters (photosynthetic rate, photosynthetic contribution rate) and biomass parameters (leaf biomass, inflorescence biomass, 1000-seed weight, seed yield) of leaves at different positions were measured during seed development of Elymus sibiricus. The aim of this study was to investigate changes in photosynthetic parameters and biomass parameters with the advance of seed development, and to determine the relationship between seed yield and photosynthetic performance of leaves at different positions during seed development. There were significant differences in photosynthetic rate and photosynthetic contribution rate of leaves at different positions at different stages of seed development, and among leaves of different positions at the same stage. Leaf biomass and leaf water content of different leaf positions changed significantly with seed development. At the flowering stage, seed yield was positively correlated with photosynthetic rate, leaf biomass and total leaf biomass in leaves at positions 1 to 5. At the seed filling stage photosynthetic rate of the first and third leaves and leaf biomass of the first two leaves were correlated with seed yield. At the milk stage, seed yield had a negative correlation with photosynthetic rate and leaf biomass from the fourth to fifth leaf. At the dough and ripe stages, seed yield had a positive correlation with photosynthetic rate of the first leaf and with leaf biomass of the first two leaves, but a significant negative correlation with photosynthetic rate from the third to fifth leaf and with leaf biomass from the third to fifth leaf. Seed yield was negatively correlated with total leaf biomass after the filling stage and with inflorescence biomass at the filling and milk stages, but a positive correlation with inflorescence biomass at the dough and ripening stages. It was concluded that crop management should promote growth and nutrient accumulation of leaves at different positions during vegetative growth to promote seed production. After the filling stage, it is very important to protect the integrity and health of the first three leaves to increase seed yield and seed quality.