It is difficult but important to confirm the order of system unidentified.Present existing methods cannot efficiently provide result and criterion to confirm the order reasonably.Rational function fit is a new polynomial fitting algorithm which was proposed by B.Gustavsen etc.The nonlinear fitting problem is changed into a linear problem by introducing the function of pole-substitution.Rational function fit gets poles and residues which are coefficients of series in modal coordinates.It will not cause redistribution of coefficient when the system is set with different model order.