The　generation　and　their　velocity　distribution　of　free　electrons　generated　in　field　ionization　process　is　presented　by　using　the　single　electron　tracking　method　in　the　interaction　of　an　intense　laser　pulse　with　noble　atoms.　Based　on　the　quasi-static　field　approximation,　the　density　of　field-ionized　free　electrons　in　the　pulse　envelope　is　obtained　by　solving　the　Ammosov-Delone-Krainov　ionization　rate　equation.　The　simulation　result　shows　that　the　laser-generated　plasma　can　rapidly　consume　the　laser　energy　at　falling　edge　of　pulse　and　change　the　pulse　shape　in　the　laser-atom　interaction.