Abstract: The once-through steam generator with double-side heat transfer component was presented in this paper. And the component is composed of straight tube outside and helix tube inside. This special structure enhances the heat transfer, but increases the flow resistance at the same time. Thus, there is an optimal problem which is to find an optimal pitch of the inner helix tube with the best effect of heat transfer and a minimum flow resistance. Based on analysis of the effects of the pitch on heat transfer enhancement and flow resistance, the pitch was optimized by the constrained nonlinear optimization method. The optimal pitches are obtained by solving the optimization problem. The conclusions can be used for engineering practice.