The power transmission system includes: a power source; a first motor generator unit; a system power output portion; and a mode conversion device, where the mode conversion device includes: an input portion, a first conversion portion, a second conversion portion and an output portion, the output portion is connected to an input end of the system power output portion, the input portion is suitable for outputting power from at least one of the power source and the first motor generator unit, the input portion is selectively connected to one of the first conversion portion and the second conversion portion, and each of the first conversion portion and the second conversion portion cooperates with the output portion to perform transmission; and the input portion is connected to the second conversion portion, so that a rotational speed output by the input portion is reduced and then output to the input end of the system power output portion.