The invention discloses an automatic transmission. The automatic transmission comprises a first simple planet row, a second simple planet row and a third simple planet row, wherein the first simple planet row, the second simple planet row and the third simple planet row are arranged coaxially, and external power is input from a first sun gear of the first simple planet row and output from a third gear ring of the third simple planet row. The automatic transmission further comprises a first brake, a second brake, a third brake, a fist clutch, a second clutch, a third clutch and a fourth clutch. Only the three non-composite simple planet rows and seven gear shifting devices (the number of the simple planet rows and the number of the gear shifting devices are equivalent to that of common five/six speed AT at present) are used for achieving 10-AT with twelve gears including ten forward gears and two backward gears. Compared with an existing automatic transmission, the automatic transmission has the advantages of being low in cost, small in speed ratio span between every two adjacent gears (which is beneficial for improving gear shifting quality), compact in structure, small in size, more suitable for being applied to FF passenger vehicles and other new types and the like.