O-shape board finite element modal in the thickness measurement system is established based on ANSYS. O-shape board topological optimal design is accomplished with reducing the volume ratio for constraint condition and the minimum flexibility of the overall structure for objective. The improved structure of O-shape board is analyzed again. Under the circumstances to meet the actual needs, the optimized structure weight is reduced and manufacture cost is decreased. O-shape board structure lightweight design is achieved. The result indicates that the proposed optimization design method is feasible.