Abstract: On July 5~6, 2008, we carried out a survey on the intensive semi-enclosed seawater shrimp ponds at Honghai Bay, Shanwei, Guangdong which had cultured shrimps for 87~88 d, monitoring the water quality every 4 h to study the diurnal variation of water quality in the late culture period and to provide theoretical reference for a reasonable, instant water quality regulation for that period.Results show that most water quality factors have an obvious diurnal variation except COD and DIN.NH₄⁺-N peaks at 3:00 and 9:00 and bottoms at 5:00 while NO₂^--N bottoms at 3:00 and 9:00 and peaks at 5:00.Both pH and DO bottom at 5:00 and peak at 13:00.Therefore, the water quality changes quickly during 3:00~9:00, and it is important to pay attention to the variation and take measures such as adding water quality regulator to increase DO value, and to reduce the production of NH₄⁺-N, NO₂^--N and their toxicity.