To study the evolution of femtosecond (fs) laser pulse in a long-distance two-level medium, the Maxwell-Bloch equations of 5 fs carrier interaeting with two-level medium are numerically solved with use of finite-difference time domain procedure. It is found that the 5 fs pulse envelope, obtained within the slowly varying envelope approximation and rotating-wave approximation, agrees nicely with the carrier field. It is also found that it takes 50000 times longer distance for a pulse with an area of 3.2π to evolve into a stable symmetric hyperbolic-secant pulse than its area varying into a stable value. The evolution process is complicated, for the pulse's spectral width much larger than the atomic linewidth. As a result, an injected pulse with an area of 3.2π could not maintain a shape of symmetric hyperbolic-secant during its area evolving into stable, and must propagate a very long distance to transform into a stable solitary wave. In addition, comparison of the results of the 5 fs and 50 fs pulse has demonstrated that the shorter the pulse is, the more intricate process, the longer distance it needs.