The invention discloses an automatic travelling system of a flaw detection trolley of pipeline welding seams. The automatic travelling system comprises a flaw detection trolley frame, two driving wheels are arranged on the bottom face of the flaw detection trolley frame and connected through a wheel axle, and the wheel axle is mounted on the flaw detection trolley frame through a suspending frame;the wheel axle is provided with a wheel axle bevel gear, and a transmission shaft capable of conducting vertical transmission is arranged on the flaw detection trolley frame corresponding to the wheel axle bevel gear; the lower end of the transmission shaft is provided with a lower transmission shaft bevel gear which is engaged with the wheel axle bevel gear, and the upper end of the transmissionshaft is provided with an upper transmission shaft bevel gear; the flaw detection trolley frame is provided with a driving motor, a rotating shaft of the driving motor is provided with a rotating shaft bevel gear, and the rotating shaft bevel gear and the upper transmission shaft bevel gear are subjected to engaging transmission; guiding rail clamping devices are correspondingly arranged on the two sides of the flaw detection trolley frame, and each guiding rail clamping device comprises an adjusting screw rod, an L sliding frame, a lower limiting wheel and an upper limiting wheel; and the adjusting screw rods are connected with the upper ends of the L sliding frames, the L sliding frames move left and right by rotating the adjusting screw rods, the L sliding frames are provided with thelower limiting wheels and the upper limiting wheels, and the lower limiting wheels and the upper limiting wheels are clamped on guiding rails.