The invention provides an automatic transmission. Due to reasonable layout, the automatic transmission can be provided with six forward gears and one reverse gear by controlling two brakes and three clutches. Only the two brakes are used by directly fixing a third sun gear on a box body, and therefore, on one hand, the structure of the automatic transmission is more compact, and on the other hand, the impact of the brakes to the box body in a gear shifting process is reduced, and the loaded condition of the box body is improved. Relatively speaking, the amount of the brakes is reduced, the amount of the clutches is increased, and therefore, on one hand, the automatic transmission can be more compact by nesting layout, and on the other hand, the utilization ratio of the clutches can be increased, an idle transfer line is disconnected or shortened, furthermore, the drag torque of an idle planet row is reduced, the churning loss of a belt row is reduced, and the transmission efficiency is increased.