Abstract: By means of curve fitting technique, the model parameter formulas of fluid lines for the two different high-frequency and low-frequency series expansions are combined according to the Bessel function into one approximation for the full frequency range (0 < ω < ∞) of continuous line parameter variation. The result is in agreement with that of Kirshnaiyer in high-frequency range (put forward in 1967), and better than that in low-frequency range. It is accurate enough for practical application.