Abstract: By imposing external influence on the Lokta Volterra predator prey system with periodic coefficients, a predator prey system with impulsive effects is obtained. Based on topological degree theory, periodic solutions for this system under impulsive conditions are studied. By reasonably increasing the quantity of preys and decreasing the quantity of predators, a prioyi bounds are obtained and a new Fredholm operator is defined. At last, a group of sufficient conditions for existence of periodic solutions are given.