For the world's first Offshore Floating Dock(OFD), the dimension control system and method to aim at managing the dimension of offshore blocks in the process of erection are developed. This paper introduces the dimension control system and deals with the method of creating a simplified OFD model and of optimizing ballast-water plan to satisfy the demanded dimension of erected offshore plant blocks. A number of tests and successful application to the Jack & Saint Malo (JSM) offshore plant project was achieved and the results are also introduced.