Abstract: To determinate the influence of aquatic environment of greenhouse pond on outbreak of tilapia motile Aeromonas septicemia(MAS), the authors investigated 95 water samples from infected ponds and 78 water samples from uninfected ponds in Beijing National Tilapia Seed Farm during the past five years. The results show that the average temperature and pH values of infected and uninfected ponds are not significantly different respectively(p＞0.05). However with water temperature increasing from 18℃ to 27℃, the outbreak percent of infected pond enhances. The average dissolved oxygen, total ammonia-N, unionized ammonia-N, nitrite-N and COD of infected ponds are (2.85±1.45)mg/L, (2.48±0.99)mg/L, (0.067±0.048)mg/L, (0.361±0.233)mg/L, (39.56±20.31)mg/L, respectively.The average dissolved oxygen, total ammonia-N, unionized ammonia-N, nitrite-Nand COD of uninfected ponds are (3.41±1.39)mg/L, (0.94±0.76)mg/L, (0.025±0.025)mg/L, (0.105±0.066)mg/L, (20.62±9.73)mg/L, respectively. Except that the dissolved oxygen of infected ponds is significantly lower than that of uninfectedponds(p＜0.05), the total ammonia-N, unionized ammonia-N, nitrite-N and COD of infected ponds are significantly higher than that of uninfected ponds(p＜0.01). The research suggests that enhanced temperature, low dissolved oxygen, high total ammonia-N, elevated unionized ammonia-N, high nitrite and exorbitant COD of greenhouse ponds lead to the outbreak of tilapia motile Aeromonas septicemia mainly by Aeromonas veronii biovar sobria.