In　order　to　find　a　more　effective　way　for　reducing　estimation　errors,　some　possible　infection　will　be　discussed　in　this　paper,　which　is　caused　by　different　development　roles,　different　data　collection　methods　and　different　data　analysis　methods.　By　three　phases　of　the　research　of　object　collection,　data　collection　and　data　analysis,　this　paper　discusses　the　reasons　for　software　estimation　error.