A tapered countersink drill bit comprising: at least two screwing knifes and at least two screwing trenches being formed between two adjacent screwing knifes ; the screwing knifes and the screwing trenches being extended from a front end of a polygonal rod body ; each screwing knife being formed by a left lateral side and a right lateral side ; a length of an edge of the left lateral side (121 A) being shorter than that of an edge of the right lateral side ; each screwing knife comprising: a front drilling head ; a tapered portion formed at a backside of the front drilling head ; an outer diameter of a front end of the tapered portion being equal to that of a rear end of the front drilling head ; a rear drilling head formed at a backside of the tapered portion ; an outer diameter of a rear end of the tapered portion being equal to that of a front end of the rear drilling head ; the outer diameter of the front drilling head being smaller than that of the rear drilling head ; wherein the screwing angle of each screwing knife is identical from the front drilling head to the rear drilling head ; and a screwing angle of each screwing trench being identical from the front drilling head to the rear drilling head ; a width between two adjacent knifes at a front end of the rear drilling head is longer than a width between the two adjacent knifes at a rear end of the rear drilling head . at least two screwing knifes and at least two screwing trenches being formed between two adjacent screwing knifes ; the screwing knifes and the screwing trenches being extended from a front end of a polygonal rod body ; each screwing knife being formed by a left lateral side and a right lateral side ; a length of an edge of the left lateral side (121 A) being shorter than that of an edge of the right lateral side ; each screwing knife comprising: a front drilling head ; a tapered portion formed at a backside of the front drilling head ; an outer diameter of a front end of the tapered portion being equal to that of a rear end of the front drilling head ; a rear drilling head formed at a backside of the tapered portion ; an outer diameter of a rear end of the tapered portion being equal to that of a front end of the rear drilling head ; the outer diameter of the front drilling head being smaller than that of the rear drilling head ; a front drilling head ; a tapered portion formed at a backside of the front drilling head ; an outer diameter of a front end of the tapered portion being equal to that of a rear end of the front drilling head ; a rear drilling head formed at a backside of the tapered portion ; an outer diameter of a rear end of the tapered portion being equal to that of a front end of the rear drilling head ; the outer diameter of the front drilling head being smaller than that of the rear drilling head ; wherein the screwing angle of each screwing knife is identical from the front drilling head to the rear drilling head ; and a screwing angle of each screwing trench being identical from the front drilling head to the rear drilling head ; a width between two adjacent knifes at a front end of the rear drilling head is longer than a width between the two adjacent knifes at a rear end of the rear drilling head . <EOS>