The propagation of ultrashort pulses in the anomalous dispersion regime o f single-mode fibers is analyzed by solving a generalized nonlinear Schrodinger equation numerically. The results show that the input pulse with limited frequency chirp always evolves into fundamental soliton which shifts continuously to lower frequency until the initial pulse peak power reaching the threshold. The soliton pulse width decreases with the input peak power but its frequency shifting speed increases with the input Peak power. Generally, the effect of initial pulse shape on the soliton generation is negligible, but with the initial chirp becomes stronger and stronger, this kind of effect will be stronger too. We also show that the effects of higher-order dispersion and self-steepening on the soliton generation are ignorable for input pulse of 1 ps width. The shaping mechanism of the principal fundamental soliton is resulted in the pulse narrowing and intrapulse stimulated Raman scattering.