We introduce inductive lightning and direct lightning that affect the state of polarization (SOP) of signal light in optical ground wire (OPGW) of power transmission line, deduce the helical component of direct lightning on OPGW, and establish the transient model of signal SOP in OPGW based on direct lightning. We also monitored actual optical transport network (OTN) systems deployed on OPGW in thunderstorm-prone areas for three months, recorded error codes and SOP transients of more than 1.5 Mrad/s using a bit error rate tester and a polarimeter accordingly. The highest SOP transient rate was up to 43 Mrad/s. Finally, we verify the accuracy of the model through simulation and experiment comparison.