The mechanism of causing the migration and accumulation of bloom-forming species is important content for the bloom occurred by cyanobacteria. In recent years, the structure of phytoplankton community has changed in Baihua Reservoir. However, there was no such information in previous literatures. This paper was based on the research of Baihua Reservoir to elaborate diel vertical migrations of the phytoplankton. It may be important to understand the mechanism of bloom and explain the regularities of the vertical migrations. The study on diel vertical distribution of phytoplankton in Baihua Reservoir was carried out from July 31 to August 1, 2012. The sampling sites was located at the center of the Baihua Reservoir. All the samples of phytoplankton were carried on qualitative and quantitative analysis in laboratory. Meanwhile, photosynthetically available radiation and water temperature were measured. Patterns of diel vertical distribution were elucidated quantitatively using two different analysis methods. Firstly, mapping method was used to describe the time-depth distribution of phytoplankton individuals and abiotic factors. Secondly, the significant differences of vertical distribution from a state of randomness were tested with Morisita's index. The correlations between Morisita's index of biological indicators and environmental factors were analyzed by Statistical Product and Service Solutions. The results showed that the number of phytoplankton species about cyanobacteria, chlorophyta and bacillariophyta was discriminable at different depths. Cyanobacteria mainly distributed from 0.5 m to 2 m, chlorophyta from 0.5 m to 6 m and bacillariophyta mainly distributed from 6 m to 14 m. Pseudanabaena limnetica was the predominant species, accounted for 82.69％of total abundance. The Pseudanabaena limnetica stratified in the 2-3 m at 8:00a.m.. The cells were tent to migrate to the surface during after 2 hours. The Pseudanabaena limnetica above the layer sink down from 10:00a.m. to 12:00a.m., appearing the maximum in 2 m. They floated up again from 12:00a.m. to 14:00, and aggregated in 1 m at 16:00. It showed that Pseudanabaena limnetica presented the vertical migration in the day. The morisita's index of Pseudanabaena limnetica was between 1.45 and 2.07 in the day, and between 1.40 and 1.46 in the night. Combined with the depth-time distribution, it showed that Pseudanabaena limnetica aggregated in water layers. And the aggregation degree was lower in the night than that in the day. Pseudanabaena limnetica could reflected the total of phytoplankton abundance. Densities of phytoplankton and Pseudanabaena limnetica were higher in the day than that in the night. The distribution patterns of phytoplankton and Pseudanabaena limnetica were coincident with the photosynthetically available radiation and water column temperature difference. Carbohydrate ballasting and gas vesicle burst might be the main mechanisms for buoyancy changes in Pseudanabaena limnetica during the period. At the same time, it was probable that the change of water column temperature difference caused passive migration of Pseudanabaena limnetica.