Abstract: On July 5~6, 2008,we carried out a survey on the intensive seawater shrimp farm 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.