The rapid estimation method of the hull girder when the main hull of Floating Production Storage and Offloading (FPSO) is integrated with the upper module structure is studied in order to estimate the deformation of the hull girder roughly in the preliminary stage of hull design. According to the characteristics of FPSO, the relevant estimation formulae at home and abroad are improved, and the calculated deformation values of the improved formulae are compared with those calculated by the whole-ship finite element method. The results show that the estimated values are in good agreement with those calculated by the finite element method. The improved formulae meet the precision requirement of the hull deformation estimation of FPSO and are suitable for rapid hull deformation estimation of FPSO.