The blow-up problem of the solutions for the nonlinear string equation with boundary input-output structure was studied.The interaction between boundary input-output structure and interior nonlinear source term was analyzed by using energy method.An auxiliary function wasintroduced.And according to the properties of the derivative of the auxiliary function

the sufficient conditions under which the auxiliary function blew up were obtained.Thus it draws the conclusion that the solutions of the system blow up in finite time if the initial energy of the system is non-positive and the input and output functions satisfy some conditions.