The invention discloses a plug-in type hybrid power transmission based on an AMT and a speed changing method, and belongs to the technical field of vehicle transmission. The plug-in type hybrid powertransmission comprises an engine, an engine normally-disengaged clutch, a motor/electric generator, a first input shaft, a second input shaft, a first middle shaft, a second middle shaft, a first output shaft, a second output shaft, a reverse gear shaft, a plurality of synchronizers and a plurality of shifting gears. The first input shaft is a hollow shaft and is connected with the engine normally-disengaged clutch. The second input shaft is a solid shaft and is connected with the output end of the motor. The first input shaft transmits power to the first middle shaft and the second middle shaft through an idle wheel. A first-gear driving gear, a second-gear driving gear, a third-gear driving gear and a fourth-gear driving gear are arranged on the first middle shaft and meshed with drivengears on the first output shaft respectively. A compensation-gear driving gear is fixedly arranged on the second input shaft and meshed with a compensation-gear driven gear on the second output shaft.The power continuity during gear shifting and the pure-electric driving are realized. The plug-in type hybrid power transmission is applied to a hybrid power vehicle and has the advantages of being good in dynamic property and economy, smooth in gear shifting, high in work efficiency and the like.