Numerical solution of the RFQ structure optimization problem requires a great amount of computation. Each consecutive step of the numerical optimization includes modification of geometry of the channel and computation of electromagnetic field for modified geometry. Therefore, a simple model describing the field in the channel is needed for the optimization. Such model is proposed in this report. It differs from the commonly used traditional model of the field, which can be applied when profiles of the vanes are described by the harmonic functions of the longitudinal coordinate. Our model is more general and can be applied for arbitrary profiles of the vanes.