Guided wave detection can be used as a promising technology of lighting strike monitoring for aircraft composites, during which the online lighting strike-induced guided wave response sensing under the harsh lightning environment remains a challenge. A high-reliability piezoelectric-fiber-optical online sensing method is studied in this paper. In the method, the lightning strike-induced guided wave responses are online sensed by PZT wafers integrated with the composite structures. Furthermore, to avoid the severe effects of lightning current coupling and EMI, the photoelectric conversion transmission is particularly adopted to transmit the guided wave responses to the following data acquisition system. The lightning strike-induced guided wave responses, with high SNR and free of crosstalk, can be thus online acquired. With the basic principle of piezoelectric online sensing firstly analyzed, the piezoelectric- fiber-optical online sensing method is developed and finally validated by a lightning strike experiment on a carbon fiber reinforced plastic (CFRP) composite plate.