In order to understand the distribution characteristics of phytoplankton at the spatial and temporal dimensions in wetlands in Qinghai-Tibet Plateau, Mitika Wetland, one of the four international wetlands in Tibet, was selected as the research area where 14 sampling points were set up along the main water system-Miti Tsangpo and its tributaries, and lakes in July and October 2019. A total of 252 water samples of phytoplankton were collected in two hydrological periods (wet season and dry season). The species of phytoplankton were identified by living observation and fixed staining method and counted by direct counting method. The dominance index (Y), replacement rate (r), niche width (B_i), niche overlap value (O_ik) and ecological response rate (R) of the dominant phytoplankton species were calculated. The niche differentiation method was used to analyze the niche of dominant species of phytoplankton at different dimensions in time and space. According to the dominance index, 8 dominant species (Y>0.02) of phytoplankton were identified among which Fragilaria crotomensis and Tabellaria fenestrata were the common dominant species in two periods, The niche width values of dominant species were quite different and can be divided into three niche groups, and the dominance index was positively correlated with the niche width. The niche overlap values of phytoplankton dominant species were high at the spatial, temporal and spatial-temporal dimensions with O_ik>0.6 accounting for 89.29%, 60.71% and 28.57%, respectively. The sum of ecological response rates was negative at the temporal dimension and was positive at the spatial and the spatial-temporal dimension indicating that the phytoplankton community was at developing stage in the wetland. RDA analysis showed that, pH, DO, total nitrogen and turbidity were the important factors affecting the distribution of dominant phytoplankton species in the Mitika Wetland, and there was obvious ecological differentiation in the dominant species of phytoplankton at these environmental factors.