An apparatus for the cutting and automatic opening of a blown film produced by means of a blow forming process, preferably by means of an extruder having an annular-shape extrusion head, and a plurality of rollers or cylinders (8, 6) for spreading a blown bubble, wherein said apparatus includes a single knife or cutting blade to perform a single longitudinal cutting for the opening of a cooled down blown film, wherein said apparatus further comprises stretching means installed in line and comprising at least one pair of transport and opening means (14, 14', 15, 15'), as well as guiding and conveying means (16, 16'), for the open film obtained by virtue of said single longitudinal cutting of the film, wherein said stretching means (14, 14', 15, 15', 16, 16') are positioned downstream of said single knife or cutting blade and are respectively arranged obliquely, horizontally and vertically on an inside of said film to stretch and open it automatically following the longitudinal cutting; for such a purpose, said stretching means (14, 14', 15, 15', 16, 16') being adapted to space apart two cut edges of the film to arrange the latter completely open on a single plane over the entire width, characterized in that said obliquely positioned stretching means consist of a pair of tubes and said horizontally positioned stretching means consist of a pair of tubes, each of said obliquely positioned stretching means being rigidly bound to a respective one of said horizontally positioned stretching means, and said horizontally positioned tubes consist of a pair of coaxial tubes, mutually slidable internally to each other, preferably by means of the manual activation of a pair of cranks, or said film stretching means consist of two rows of rotary shaped rollers, arranged in sequence in the motion direction of the film and transversely thereto, configured to allow the film itself to freely slide, for opening and stretching it, respectively. wherein said apparatus includes a single knife or cutting blade to perform a single longitudinal cutting for the opening of a cooled down blown film, wherein said apparatus further comprises stretching means installed in line and comprising at least one pair of transport and opening means (14, 14', 15, 15'), as well as guiding and conveying means (16, 16'), for the open film obtained by virtue of said single longitudinal cutting of the film, wherein said stretching means (14, 14', 15, 15', 16, 16') are positioned downstream of said single knife or cutting blade and are respectively arranged obliquely, horizontally and vertically on an inside of said film to stretch and open it automatically following the longitudinal cutting; for such a purpose, said stretching means (14, 14', 15, 15', 16, 16') being adapted to space apart two cut edges of the film to arrange the latter completely open on a single plane over the entire width, characterized in that said obliquely positioned stretching means consist of a pair of tubes and said horizontally positioned stretching means consist of a pair of tubes, each of said obliquely positioned stretching means being rigidly bound to a respective one of said horizontally positioned stretching means, and said horizontally positioned tubes consist of a pair of coaxial tubes, mutually slidable internally to each other, preferably by means of the manual activation of a pair of cranks, or said film stretching means consist of two rows of rotary shaped rollers, arranged in sequence in the motion direction of the film and transversely thereto, configured to allow the film itself to freely slide, for opening and stretching it, respectively.