The iterative optimization method of using equivalent model is used to design launch vehicle payload bearing structure—composite cross beam. The strategy is based on the idea of the substitution of equivalent models for detailed models. Using the engineering software Patran/Nastran, optimization finds geometry of cross-sectional with a minimum weight and stacking sequences satisfying laminate design rules. Equivalent model method can decrease numbers of design variable as well as ensures reliability of result. The strategy showed weight savings of 38% compared with the initial design with little computational effort. This has demonstrated that the strategy appears efficient, reliable and extendable into the cross beam structure.