Abstract: Through analyzing water quality change trend in Luanhe River Channel since the construction completion,the main area with high pH value was found among Yuqiao Reservoir.The causes of effecting the pH value increment were discussed from various aspects,e.g. the rock weathering and soil action influencing the solute in the surface water,the groundwater moving,and the phytoplankton photosynthesis.It was determined that photosynthesis of algae is the principal reason of the pH value increment.High concentration of nitrogen and phosphorus in the reservoir results in algae bloom.Algae absorb CO₂ and release O₂,breaking out the balance between HCO^-₃,CO₂ and OH^- and bringing about the pH value increment in the reservoir.