A numerical method is presented to calculate electric field, potential and motion trajectory of molecules in a hexapole for rotational state-selection and orientation of polar molecules. Potential distribution obeys Lapace equation, by which numerical distribution by points is obtained. Then points are fitted to analytical expressions of potential by lest square fitting. The coefficients in a finite-term series approximation for potential distribution are determined. Electric intensity is obtained by differential of the potential. In hexapole, classical trajectories of polar molecules are described by Newton's equations and solved with four order Runge-Kutta method. The rotational energies and Stark energies are solved by quantum mechanics. With electric field distribution formula, influences of field distribution on hexapole rotational state-selection and orientation of N₂O molecules are discussed.