The invention provides a nine-gear planetary automatic transmission of a vehicle drive system, which comprises planetary lines, input components, output components, transmission components, brakes and clutches. The input components and the output components are arranged on the two ends and are respectively connected with an input shaft and an output shaft, and four planetary lines, five transmission components, four brakes and two clutches are used. Compared with the prior art, due to adopting the four planetary lines, four brakes and two clutches with an additional forward gear, the invention realizes the purpose of nine forward gears with one brake reduced, thereby reducing the weight and cost of the transmission. Particularly, the four planetary lines are all single-star simple planetary lines, and the gear parameters of the planetary lines have two pairs of identical K values, further reducing the types of gears, and simultaneously obtaining the transmission ratio property with the optimal overall performance.