The invention discloses a four-gear mechanical automatic transmission power assembly system based on planetary gear mechanisms. The four-gear mechanical automatic transmission power assembly system comprises an engine, a main clutch, a motor, a first planet row, a second planet row, two engagement devices, three auxiliary clutches, an electromagnetic brake, two brakes, a gear ring, and a transmission input shaft as well as a transmission output shaft; the first planet row, the second planet row and a splined hub are orderly arranged on the transmission input shaft; the splined hub is engaged with an engagement sleeve; and switching of four forward gears, a neutral gear and a reverse gear is realized by means of mutual cooperation of the engagement sleeve, the electromagnetic brake and the clutches. Compared with a traditional mechanical transmission, the four-gear mechanical automatic transmission power assembly system based on the planetary gear mechanisms is provided with no synchronizer, and instead, the motor is adopted for accurate speed regulation to realize smooth gear shifting, and two planetary gear mechanisms are used to take the place of traditional a plurality of pairs of gear mechanism; as a result, the overall mechanism is simple, small in volume, low in cost and easy to manufacture.