Based on the statistical data of the unit weight of fixed jacket center platform, the factors affecting the unit weight of the two block structures are given by numerical analysis method. The lightweight of block structure is analyzed and demonstrated from three aspects: the corresponding relationship between member section and bearing capacity, member stress level and overall structure scheme. Several methods to optimize the weight of offshore platform block structure are proposed. The size ranges of welded H-shaped steel section and welded structural steel pipe are recommended for platform designer.