Conventional FDSO (Floating Drilling Storage Offloading) construction method which needs particular dock and berth has limitation in application. A new block construction method for FDSO without dock and berth is offered in this paper based on the analysis of present FDSO block construction method. In this method, semi-submersible barge is utilized as a closure construction platform. The process includes the following stages: pre-segmentation closure, set limit and positioning components, separate the block from semi-submersible barge after block launching, rotate block with 90 degree and complete block construction. With the advantage of high assembly precision and small flatness adjustment, the presented method is especially suitable for cylinder FDSO’s close construction without large dock and berth.