An SI epidemic model with stage structure is investigated. In the model, impulsive biological control is taken, that is, we release infected pests to the field at a fixed time periodically. We get a sufficient condition for the global asymptotical stability of the pest-eradication periodic solution (0,0,I˜(t)), and a condition for the permanence of the system. At last, a brief discussion shows that our results will be helpful for pest management.