The invention discloses a power transmission system and a vehicle thereof. The power transmission system includes: a power source; a first motor generator unit; a system power output part; a mode conversion device, and the mode conversion device includes: an input part and a planetary gear mechanism , an adapter and an output part, the output part is connected to the input end of the power output part of the system, the input part outputs the power from the power source and the first motor generator unit, the planetary gear mechanism includes first to third elements, the first The element is fixed to the input part, the second element is fixedly arranged, and the adapter is used to selectively engage one of the first element and the third element with the output part, and the output part is engaged with the third element, so as to be suitable for making the input part output a high speed The speed at the input end of the power output part of the system. In this way, the driving modes of the vehicle can be enriched, and the vehicle can adapt to different road conditions.