A method of determining three parameters of a separatrix shape on a field-reversed configuration plasma is described. The three parameters are center radius a, length b (half-distance between X-points), and shape index n when the separatrix radius rs is approximated by the radial position derived from the relationship r2/a2+|z|n/bn=1. To demonstrate the details of the method, magnetic fields in a vacuum region around the separatrix are numerically calculated for the assumed separatrix shapes in the ranges 4≤b/a≤10 and 1.2≤n≤4. Excluded flux radii rΔϕ(z) along the plasma are obtained using the calculated magnetic fields. First, it is confirmed that rΔϕ(0) at the axial midplane can be replaced by a with good precision. Second, a trial function including rΔϕ(z) and parameter n′ is defined. The parameters b and n are determined using the trial function that becomes b without any axial dependence when rΔϕ(z)≈rs and n′≈n. Errors in b and n arising from edge-layer plasmas are quantitatively estimated by introducing its width in the trial function. Finally, it is demonstrated, using experimental data, that the separatrix parameters are easily determined by the present method.