To optimize the structure of posed direct-decoupling planar capacitive sensor (PCS), methods for capacitance and sensitivity computation are studied, and the structural optimization is established. Firstly, four methods are compared with to find a suitable analytical method for the calculation of posed PCS capacitance and sensitivity. Then, electrostatic field equation method are applied and under the condition of given sensitivity, PCS structural parameters, i.e. the length of side and the number of plates is optimized. Computation results indicate that electrostatic field equation method is available for the calculation of capacitance and sensitivity on one hand, and on the other hand, when the length of side of PCS plate is twice the gap of two parallel part (l≈2d )and the number of plates is seven (N=7), PCS can meet sensitivity requirement and take up the smallest dimension.