The　master　equation　of　the　car　cluster　in　freeway　traffic　is　reinvestigated　on　the　basis　of　the　linear　assumption　of　the　transition,　in　which　the　dynamic　behavior　of　the　cluster　in　a　one-lane　freeway　traffic　flow　is　studied.　The　expression　of　the　mean　size　of　the　cluster　versus　t　is　derived　in　term　of　the　solution　of　the　birth-death　equation.　We　also　obtain　the　analytical　expression　of　the　probability　distribution　P(n,t).　Numerical　simulations　testify　the　results　derived　as　well.