The invention discloses an automatic flaring device for round PVC pipes. The automatic flaring device comprises a case fixed to the ground; at least one pair of inclined holes are formed in the side wall of the case, and the ends, located in the case, of the inclined holes are lower than the ends, located outside the case, of the inclined holes; a lead screw rotating in a limited manner is arranged in the center of the case, and a motor driving the lead screw to rotate is arranged at the bottom of the lead screw; an internal thread sleeve is arranged on the lead screw, and a pair of inclined sliding grooves are formed in the outer side of the internal thread sleeve; one sliding groove corresponds to one inclined hole, and the sliding grooves are perpendicular to the inclined holes; and telescopic guide columns are arranged in the inclined holes, movable sliding blocks embedded in the corresponding sliding grooves are arranged at the ends, located in the case, of the telescopic guide columns, and supporting plates for opening pipe openings are arranged at the ends, located outside the case, of the telescopic guide columns. According to the automatic flaring device, the sliding grooves can be driven to move through the rotation of the lead screw, so that all the telescopic guide columns stretch out and draw back synchronously, and the effect of performing automatic flaring on thePVC pipes together is achieved. The automatic flaring device is simple in structure, high in flaring synchronization rate, adjustable in speed, high in flaring precision and convenient to use instantly on a construction site.