Abstract
The separation of variables for x- and tn-constrained flows of some kind of soliton hierarchy is shown. Since each equation in soliton hierarchy can be factorized into two commuting x- and tn-constrained flow, the separability of x- and tn-constrained flow provides the Jacobi inversion problem for soliton equations which is solvable in terms of Riemann theta function by standard Jacobi inversion technique. We use Kaup-Newell hierarchy to illustrate the method.