Litopenaeus vannamei GILI tilapia enclosure polyculture experiment was conducted in Dianbai, Maoming city, Guangdong province from June to August, 2011. Selecting an experiment pond, and setting up 24 enclosures. These enclosures were divided into six groups (A, B, C, D, E, F), each group has four parallels, among these groups F was control group, not stocking tilapia. During aquaculture time, investigations of water quality factors and phytoplankton community were conducted .The results showed as follows: (1) Throughout the culture period, dissolved oxygen content was close in all groups. Nitrite and ammonia concentration showed an increasing trend in each group, and among them, C and D groups were significantly lower than A, B, E and F groups (P<0.05). The concentration of COD and TOC were experimental groups higher than control groups. In the experimental groups, A and B groups were higher than C, D and E groups. As for the increase amount of TN and TP, group A is highest, control group F is lowest, while in experimental groups, A and E were higher than B, C and D groups. (2) 36 phytoplankton species were identified during the culture period. Dominant species is single and prominent. Experimental groups Chlorophyll a content were higher than control group, and between the experimental groups, A and B were higher than other three groups. These results indicate that polyculture appropriate density of tilapia in shrimp ponds is conducive to improving water quality, and can regulate and control the type and quantity of phytoplankton.