It is difficult to resolve the problem of sludge bulking. The A/O process was applied for the food wastewater treatment. It is found that sludge would expanded in the low temperature. The water quality was analyzed, and 16SrDNA clone library was built. Through the analysis the microbial community, the relationship of sludge bulking and process control was studied. It showed that the percent of Proteobacteria was 63.51%, Chloroflexi was 16.22% and Thauera sp. was 20.27% in anaerobic sludge bulking. The percent of Proteobacteria was 36.08%, Chloroflexi was 24.74% and Bacteroidetes was 7.22% in aerobic sludge bulking. In the same time, Uncultured Nocardia, Uncultured Leptolyngbya sp., Thauera sp. were found which are related with sludge bulking. The low temperature was the main external ecological factors. There are several reasons to cause the sludge bulking, such as the lack of anaerobic degree in anaerobic period , poor biochemical, high concentration of heavy metal and oxygen deficiency in aerobic period. It is showed that sludge bulking would be control effectively when we regulated the process of inlet temperature, improve the anaerobic degree and aerationSfluxes.