The source region of the Yellow River is located in the eastern region of the Qinghai-Tibet Plateau andthe ecological environment in the basin is fragile. The climate in this area is classified as plateau sub-cold zone,semi-humid and plateau temperate zone, semi-humid. The annual rainfall is approximately 300 mm, and theaverage annual water temperature is 3.4‒7.2 ℃. As the global climate is changing and human activities areincreasing, the ecological environment of the basin is confronting great pressure. The protection and restoration ofthe environment around the Yellow River has become the focus of human society. To explore the characteristics ofthe phytoplankton community structure and its relationship with environmental factors, two comprehensiveinvestigations were undertaken in the main stem and tributaries of the Yellow River in the spring and summer of2023, respectively. The total length of the survey site of the river was 1200 km. Phytoplankton samples werecollected from 33 survey stations along the source region of the Yellow River. The environmental parametersincluding water temperature, dissolved oxygen level, conductivity, turbidity, pH, chlorophyll a level, suspendedsolid levels, flow velocity, river width, transparency, altitude, latitude, and longitude were collected in the field.One liter of water was collected in a polytetrafluoride bottle and returned to the laboratory for analyses of waterquality indices, including total nitrogen and total phosphorus levels and the potassium permanganate index. Intotal, 235 species of phytoplankton belonging to seven genera were identified, with an average density andbiomass of 182.75×10⁴ cells/L and 55.35×10⁻³ mg/L, respectively. Diatoms were the dominant species, accountingfor 72.31% of all species. Seasonal changes in species composition and abundance existed, but were notsignificant. From the perspective of spatial dynamics, the phytoplankton species and their abundance in the mainstem proceeded to the tributaries. The number of phytoplankton species was highest in the downstream area, andthe average density and biomass of phytoplankton were highest in the middle reaches. The Shannon-Wienerdiversity index, Margalef’s richness index, and Pielou’s evenness index values were 2.87±0.66, 6.65±0.94, and0.77±0.17, respectively. Biodiversity analysis showed that the diversity index values of the phytoplanktoncommunity were higher in spring than in summer, and these seasonal differences were highly significant. Therewere nine dominant species of phytoplankton, mainly belonging to Bacillariophyta. The dominant species in bothspring and summer were Fragilaria intermedia, Fragilaria biceps, Synedra ulna, Diatoma moniliforme, and Cymbella turgidula. Correlation analyses between the phytoplankton community structure and environmentalfactors showed that the influencing factors of the phytoplankton community structure were different in spring andsummer in the source region of the Yellow River. The main factors affecting phytoplankton density and biomass inspring were altitude, river width, and flow velocity, while in summer the main factors were the potassiumpermanganate index value, chlorophyll a level, suspended solid level, and transparency. The results of this studyprovide an important foundation for scientific decision-making regarding basin ecological environment protectionand management in the Yellow River.