Abstract: By applying an orthogonal expansion technique in random space,a general formula for an expanded order system is established. This formula can be used for various multi-degreeoffreedom stochastic structural systems which possess random mass, damping and stiffness parameters. For the continuous random field, a two-step method is suggested to convert it into an expression of independent random variables.A subjunctive structure method is employed to form a basic matrix for the expanded order system.The proposed method is validated through an analysis of simulated data. Finally,the difference between the mean response of a stochastic system and the response of the corresponding "mean parameter System" is discussed.