The invention relates to a directional windowing horizontal drilling device. Multiple flexible straightening units are connected between a drill bit and a flexible drill pipe. For each flexible straightening unit, a body sleeves a hollow inner core, a water passage is formed in the center of the inner core, a spherical protrusion is arranged on the outer wall of the inner core, multiple small steel balls are installed on the surface of the spherical protrusion, the small steel balls are evenly arranged along the circumferential surface of the spherical protrusion, the bottom of the spherical protrusion is contracted to a straight surface, and a tight fit can be formed between the straight surface and the body; the top end of the body is sleeved with a pressure cap, the body and the pressure cap are provided with arc concave surfaces matching the spherical protrusion on the inner walls thereof, the spherical protrusion is right in the arc concave surfaces, strip-shaped grooves are formed in the center of the arc concave surfaces, the small steel balls are right in the strip-shaped grooves, and the length of the strip-shaped grooves is equal to the length of the bottom straight surface of the spherical protrusion; and the body and the pressure cap are pressed with a large spare cap, and the bottom end of the body is pressed with a small spare cap. The directional windowing horizontal drilling device can ensure that the trajectory of a horizontal wellbore is kept straight in the process of drilling.