Abstract: Uncertainty in maritime structure ultimate strength is induced by the variability of the scantlings, materials performance, and other factors. The ultimate strength of a ship stiffening plate with initial imperfections was calculated by the progressive collapse theory using nonlinear finite element methods. An uncertainty analysis model was established by Monte Carlo simulation and fitting technology, and was used to discuss the factors influencing ultimate strength, including the random variables of structural scantlings, material parameters, and initial imperfections. In order to obtain an accurate stochastic law with small computational cost, the 3-parameter log-normal fitting method, based on inverse First Order Reliability Method (FORM), was presented and compared with the traditional method. The results indicated that the 3-parameter log-normal method can predict ultimate strength efficiently and accurately, so it can be the basis of ship safety analysis.