【Objective】To explore the source-sink relationship in buckwheat（Fagopyrum esculentum Moench). 【Method】The common buckwheat cultivar Fengtian 1 was selected as material. Treatment(cutting the unopened flowers and inflorescences) was conducted at the 7th day after flowering, and non-treatment was used as control. The major agronomic characteristics and metabolite contents were measured, and the grain filling process was evaluated with Richards mathematical equation. 【Result】There was no significant difference in the contents of soluble sugar and chlorophyll between sink-limiting treatment and control during the early-middle grain filling, as well as the plant height, branch number, node number at maturity. The carbohydrate accumulation was accelerated during the early-middle grain filling under sink-limiting treatment. However, there was no significant difference in the grain number per plant and grain weight per plant between sink-limiting treatment and control. Compared with control, the yield of treatment significantly decreased. The determination coefficients (R²)of treatment and control were 0.996 and 0.992 respectively, which showed that the Richards growth curve well reflected buckwheat grain filling process.【Conclusion】The Richards growth curve is suitable for estimating buckwheat grain filling process. The insufficient source or unreasonable distribution of photosynthate is an important limiting factor for buckwheat yield improvement. Adjusting photosynthate synthesis and allocation should be effective to increase buckwheat yield.