On November 21，2018，an outbreak of African swine fever（ASF）was reported in Hecheng District，Huaihua City of Hunan Province by Ministry of Agriculture and Rural Affairs. In order to find out the sources of the outbreak，an epidemiological investigation was carried out by Huaihua Animal Disease Prevention and Control Center. Based on the risk factors including live pig transportation，feed，swill and bio-safety，and according to laboratory detection results of serum，feed and swill from farms related to epidemiology，associated slaughterhouses and frozen pork products from freezers，the possible pathways of ASF spreading were comprehensively analyzed. The results showed that pigs in the affected farm were fed with purchased swill in October 2018；the positive rates of ASF virus（ASFV）nucleic acid in swill and frozen pork products were 0.9%（3/348）and 25.6%（10/39）respectively，while all the serum and feed samples were negative；samples from freezers，restaurants and sale markets were detected ASFV-positive，with the positive rate of 16.7%，1.8% and 7.7% respectively. As it was investigated that the period of feeding with swill and the date of outbreak were consistent with the incubation period of ASF，and positive ASFV nucleic acid was detected in pork products that were stored in the freezer of a transportation company related to swill. In conclusion，the ASF outbreak was possibly caused by feeding with swill. Based on the above investigation results，the outbreak was effectively controlled by a series of control measures，including strict supervision over the movement of live pigs and their products，implementing the ban of feeding with swill，and strengthening management on bio-safety.