Considering the status of lacking a complete lightning category system, a comprehensive study of lightning strokes according to lightning hazard patterns is conducted, and indirect lightning strokes are divided into two categories. The concept of lightning potential difference effect is presented. Lightning counterattacks, lightning ground potential ascending, and lightning side flashovers are described, and touch voltage and step voltage are analyzed separately. The protection methods of lightning potential difference effect are introduced and the explanation of safe distance is given. The relationship between lightning potential difference effect and personal injury is also studied. It is concluded: lightning potential difference effect includes lightning counterattacks, lightning ground potential ascending, lightning side flashovers, touch voltage and step voltage related with personal injury; the protection methods of lightning potential difference effect include avoiding, bonding and maintaining safe distance.