Drivers　are　used　to　steering,　braking　or　steering-braking　when　they　meet　obstacles　in　order　to　protect　lives　and　properties.　Frecord　data　collecting　system,　dynamic　GPS　and　sensors　of　braking　force　and　angles　are　utilized　to　study　the　changing　rules　of　drivers＇　braking　velocitiy　according　to　different　time,　the　relation　between　maximum　braking　velocity　and　the　distance　between　vehicle　and　obstacle,　as　well　as　driving　velocity.　The　results　show　that　drivers＇　braking　operation　includes　four　periods　and　also　indicate　that　the　drivers＇　maximum　braking　velocity　is　changing　with　distance　in　negative-power,　however　drivers＇　maximum　braking　velocity　is　changing　with　driving　velocity　in　positive-linear.　The　regressive　formulas　are　given.　This　research　can　help　us　to　understand　drivers　behavior　under　emergency　and　also　offer　theory　basis　for　drivers＇　safety　management.