In order to explore the ecological niche and interspecific association of dominant phytoplankton community, we selected the middle and lower reaches of Lhasa River and its tributaries as research sites. Seventeen sampling stations were set up in May 2015, August 2015 and November 2016, and water samples were collected in three hydrological periods. A total of 393 phytoplankton species were identified, belonging to 100 genera, 48 families, 8 classes, 24 orders and 6 phyla. The dominance index (Y) of the dominant species was calculated, and the niche width, niche overlap, ecological response rate, total linkage, χ² test and common occurrence percentage were used to analyze the niche and interspecific linkage of the dominant species. The results showed that the whole phytoplankton community was diatom-green algae-cyanobacteria. There were 19 dominant phytoplankton species (Y>0.02), all belonging to diatoms, among which 7 dominant species such as Synedra acus and S. amphicephala were common dominant species in three hydrological periods. The occurrence frequency and dominance degree of dominant species were different in different hydrological periods. The niche width of the dominant species varied from 0.089 to 0.751, and the niche width values of S.amphicephala were all larger than 0.5 in the three hydrological periods, showing strong ecological adaptability and belonging to a wide niche species. The species of Achnathes exigua were highly competitive and their habitats tended to be specialized, which could be used as an indicator of pollution in the middle and lower reaches of Lhasa River. The niche overlap values of the 19 dominant species ranged from 0.001 to 0.966, and Diatoma aulgare occupied similar habitat resources in wet season, showing competitive interference. The niche overlap values were higher in normal seasons than in dry seasons, and the niche overlap degree was not high in general. The commonness of the use of resources is small; The relative resource occupancy and niche width of dominant species in different hydrological periods were basically the same, and most of the algae species with higher resource utilization had stronger development space. According to the analysis of interspecific association, the association of dominant species decreased gradually with the change of seasons, and the interspecific relationships of species in the community changed from close to loose from normal to wet to dry seasons, showing a trend of reverse succession. The associations of most species pairs did not reach the significant level (χ²>3.841), and the interspecific association was not close. The relative dynamic balance between dominant species and environment was not reached.