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(6×6×2m). These enclosures were divided into six groups (A, B, C, D, E, F), each group has four parallels, among these groups, A, B, C, D and E was experimental group, stocking tilapia 4, 8, 12, 18, 24 tail in each enclosure, respectively, F was control group, not stocking tilapia. Experimental groups and control group all stocking shrimp 3000 tail each enclosure. 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 group. Nitrite and ammonia concentration showed an increasing trend in each group, among them, C and D group were significantly lower than A, B, E and F group (P<0.05). The concentration of COD and TOC were experimental groups higher than control groups. In the experimental groups, A and B group were higher than C, D and E group. 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 three groups. (2) 36 phytoplankton species were identified during the culture period. Dominant species is single and prominent. Phytoplankton community structure were better in C and D groups than A and B groups. Experimental groups Chlorophylla content were higher than control group, 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, regulate and control the type and quantity of phytoplankton.