After the small intestines of pig were treated by ultrasonic treatment, heating and 5% acetic acid extraction, antimicrobial peptides were isolated from the intestine of pig by Sephadex G100 and Sephadex G25 gel filtration chromatography. The samples were analyzed by Tricine SDS polyacrylamide gel electrophoresis. The results showed that an antimicrobial peptide was purified and its molecule weight was about 5 972 Da. The antimicrobial peptide, named PSIP, strongly inhibited Escherichia coli ATCC84008, Staphylococcus aureus ATCC25923 and Pseudomonas aeruginosa ATCC27853, and neutralized chicken infectious bronchitis virus H120.