Abstract: Flowers of four-year-old Rheum tanguticum plants were labeled with cards on Flourish-blossom Day in Zhuoni county in the Gannan Tibetan autonomous prefecture, Gansu province. The cards were hung on inflorescences with consistent development signs. Seed filling dynamics were measured once every three days during filling duration, starting 10 days after the experiment’s launch. Germination traits were measured after air drying in order to explore the relationship between seed filling dynamics and germination, providing a theoretical and technical basis for standardizing seed harvesting. The 100-seed dry weight fitted to a logistic equation, with rapid increases during 12-39 days after blossoming and finishing on the 60^th day. The seeds’ water content decreased during the seed filling process. Seeds harvested on the 22^nd day following blossom showed low germination ability. Following this point in time, germination quality significantly improved and remained stable until 49 days after blossom. Water content in the seeds decreased to below 10% from 55 days after blossom. Germination quality indicators were significantly and positively related to the 100-seed dry weight and seed filling duration, but were negatively related to the water content in seeds. The seed filling rate was significantly affected by the weather. The study thus showed that seed maturity affects germination quality. The best harvest time is 55-60 days after blossom (in late July and early August), when seed water content is approximately 10%.