Lightweight technology is an important development trend of ships. Lightweight hull structure design may lead to novel designs that break through existing standards, including new structures or new materials. Based on an overview of the existing safety assessment system for ship structures, it focuses on the safety assessment process and methods for the lightweight structural design, including alternative design process, novelty evaluation indicators and approaches, and methods for determining structural safety assessment criteria based on limit states. The proposed novelty evaluation and safety assessment methods for the lightweight structural design are then applied for practical analysis of the typical cases of the lightweight structural design for oil tankers, bulk carriers, and container ships from literatures published at home. The proposed novelty evaluation and safety assessment methods for lightweight structural design can provide approaches and methods for the structural safety assessment of the corresponding new or alternative designs.