Abstract: 【OBJECTIVE】The antibiotic is a kind of secondary metabolite which is produced by the certain microorganisms in the process of growth.It has the ability that can suppress or kill some other microorganisms. The effect of ampicillin to different bacteria and different concentration of antibiotic to the same bacteria can be determined according to tength of the Bacteriostasis belt, and preliminary judge its antibacterial spectrum . 【METHOD】Using the law of filters the paper to mensurate the antibacterial spectrum of ampicillin.Colon bacillus ,staphylococcus aureus,bacillus are used.Along with ampicillin density increased.【RESULTS】 the longth of bacteriostasis belt grows gradually,and the effection of bacteriostasis become more strong.But the difference of Suppression effect that the same ampicillin to colon bacillus staphylococcus aureus,bacillus is not very certion.Throng restrain and transfers the peptide.【CONCLUSION】 ampicillin make bacteria die with cell wall disintegration.