Submarine cables is very expensive，meanwhile the construction and maintenance is complicated． Once failure occurs，the consequences are serious． The aging and breakdown of cables caused by overvoltage are the important factors leading to cable fault，so it is of great significance to study the overvoltage of submarine cable system． Based on Zhenhai － Zhoushan 500 kV AC submarine cable project，simulation models for overvoltage calculation of submarine cable is established in ATP， the transient overvoltage，switching overvoltage and lightning overvoltage are analyzed and calculated，and finally the protective measures are given． Based on the simulation results，the insulation coordination of submarine cable system is studied．