We are concerned with the nonlinear second-order impulsive periodic boundary value problem u ' ' ( t ) = f ( t , u ( t ) , u ' ( t ) ) , t ∈ [ 0 , T ] ∖ { t 1 } , u ( t 1 + ) = u ( t 1 − ) + I ( u ( t 1 ) ) , u ' ( t 1 + ) = u ' ( t 1 − ) + J ( u ( t 1 ) ) , u ( 0 ) = u ( T ) , u ' ( 0 ) = u ' ( T ) , new criteria are established based on Schaefer's fixed-point theorem.