This letter presents a low-complexity redecoding-based error-floor lowering technique for quasi-cyclic low-density parity-check codes, where a predetermined set of variable nodes are attenuated before the redecoding. Using a two-stage off-line search, the attenuation set is determined based on the error patterns collected from the standard decoding simulation. It is shown that the error floor can be effectively lowered, and only a negligible amount of complexity is introduced.