Abstract: A method of sensitivity analysis for random responses of offshore platforms under wave forces is proposed in the paper. Based on a deterministic approach the Pseudo Excitation Method (PEM), the sensitivity analysis of random responses is transformed into stationary harmonic response analysis. Furthermore, the sensitivities of the statistic properties, such as the standard deviations, the mean values and the covariance of the random responses are obtained. The sensitivity equations are solved by utilizing directly the program module of response analysis, avoiding the difficulty in calculating derivatives of eigenvectors and enhancing the computational efficiency. The comparison with the finite difference method verifies the accuracy and efficiency. Based on the sensitivity analysis method, the design optimization of offshore platform structures subject to random wave loads with size and shape design variables is investigated. The sequential linear programming algorithm together with some modification techniques, such as the adaptive move limit, the approximate line search and the approximate reanalysis, is used to solve the optimization problem. The proposed method has been implemented in the finite element analysis and design optimization software - JIFEX. An offshore platform structure is studied to demonstrate the effectiveness of the method.