There are many water quality parameters in aquaculture, the relationship of which is complicated. To find a few monitoring parameters is practical demand for farmers. Factor analysis approach is a statistical method that uses a few factors to replace many original factors. Water quality in tilapia ponds based on biofloc technology was evaluated using this method in this paper. Tilapias were kept in 12 outdoor cemented ponds. The control group was fed with commercial feed normally, while group A, B and C were fed with 100%, 80% and 75% of normal amount of feed and added with wheat starch as carbohydrate. Water samples were taken to measure pH, DO, T, secchi disk depth(SD), total ammonia nitrogen (TAN), nitrite nitrogen (NO₂^--N), nitrate nitrogen (NO₃^--N), total nitrogen (TN), total phosphorus(TP), chlorophyll a (Chl.a), floc volume(FV), total suspended solids(TSS) and total heterotrophic bacterial (THB)every week. The results showed that the communalities of T, pH, TSS and THB were lower than 0.5, and removed when factor analysis was done. Three main factors affecting the water quality were extracted when dissolved oxygen was sufficient. The first main factor was composed of TP, Chl.a and TN, which reflected organic nutrients. The second main factor was composed of TAN, SD,NO₃^--N and NO₂^--N, which reflected inorganic nitrogen and SD. The third main factor was composed of FV. The component score of each pond was calculated by the weight of 3 factor scores. The component score of group C was the lowest, and lower than the control group significantly. It showed that water quality of group C was the best of all, and biofloc technology could improve water quality and promote the sustainable development of tilapia culture. TN, TP and Chl.a should be monitored emphatically in tilapia culture systems using biofloc technology and TAN,NO₂^--N should be monitored regularly.