A power transmission system includes: a power source; a speed change unit, where the speed change unit is selectively power-coupled to the power source; a first motor generator unit; a system power output portion; and a mode conversion device, where the mode conversion device includes: a conversion device input portion and a conversion device output portion, the conversion device input portion outputs power from at least one of the power source and the first motor generator unit, the conversion device output portion is connected to an input end of the system power output portion, and the conversion device input portion is selectively power-coupled to the conversion device output portion; and when the conversion device input portion is power-coupled to the conversion device output portion, the rotational speed of the conversion device input portion is greater than or equal to the rotational speed of the conversion device output portion.