By employing a full-order dispersion model, we numerically investigat the filamentation of few-cycle laser pulse focused in argon. The focusing effect caused by lens is simulated by quadratic phases e－ik(ω)r2/2f and －ik(0)r2/2f, separately. We find that it is inaccurate to simulate the focusing effect by employing e－ik(0)r2/2f, especially for the propagation of few-cycle. So quadratic phase e－ik(ω)r2/2f must be used in order to accurately simulate the effect induced by lens.