The purpose of this study was to find out the meteorological and hydrological status and the trend through the results of the water quality monitoring network of Okjung reservoir at Jeollabuk-do. By analyzing the water quality, algae and statistical analysis, we tried to find out factors influencing the phytoplankton growth. As a result of the water quality monitoring network, The concentration change in the river monitoring network was greater than that in the lake because it was affected by rainfall directly. T-P and Organic matter concentration showed similar tendencies because particulate phosphorus flow into water body with pollutants due to rainfall. The water samples were collected from three points in the river and five points in the lake in 2017 and 2018. N/P ratio was observed to be P-limited condition in both the monitoring network and the water quality analysis. The phytoplankton community was found to be mainly dominated of diatoms during the study period. Principal component analysis of 2017 and 2018 identified 4 component that explained 74.6% and 78.3% respectively. During the study period, these components were attributed to change of meteorological and environment factor, growth of phytoplankton by inflow of organic matter and particulate phosphorus, concentrated precipitation in the summer, environmental changes causing growth of cyanobacteria. As a result of this study, T-P management is necessary because it acts as a limiting nutrient among water quality factors. And the upstream inflow river has a large change in the concentration of nutrients and organic matters entered as the flow rate changes. Therefore, it is necessary to control the growth of algae by managing the water quality of upstream inflow river.