The seawall flood defense criteria both at home and abroad have been investigated and discussed. The failure of seawall is referred to six typical modes and the inferences of the wind wave and tide are analyzed closely on each mode. A rough estimate is suggested for encounter probability of wind/wave with highest tide. A three level criteria for seawall design is proposed, which comprehensively integrates the encounter probability, seawall structure performance and usage requirements and the potential risk. Based on the above-mentioned criteria an optimization design scheme of risk principle for seawall is provided.