It is difficult to resolve the problem of sludge bulking. The A/O process was applied for the food wastewater treatment. It was found that sludge expanded in the low temperature. The water quality was analyzed, and 16S rDNA 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 is 63.51%, Chloroflexi is 16.22% and Thauera sp. is 20.27% in anaerobic sludge bulking. The percent of Proteobacteria was 36.08%, Chloroflexi was 24.74% and Bacteroidetes is 7.22% in aerobic sludge bulking. In the same time, Uncultured Nocardia, Uncultured Leptolyngbya sp., Thauera sp. are found which are related with sludge bulking. The low temperature is 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 will be control effectively when the process of inlet temperature was regulateed, improve the anaerobic degree and aeration fluxes.