A power transmission system includes: a power source; a first motor generator unit; a system power output portion; a first and a second mode conversion devices. The power source and the first motor generator unit is connected to or disconnected from the system power output portion through the first mode conversion device. The power source can be connected to or disconnected from the first mode conversion device through the second mode conversion device. The second mode conversion device includes a conversion device input shaft, a first conversion device intermediate shaft and a conversion device output shaft, and the conversion device output shaft is connected to the first motor generator unit; and power from the power source is suitable for being decelerated sequentially through the conversion device input shaft, the first conversion device intermediate shaft and the conversion device output shaft and then output to the first mode conversion device.