Provided are a system and a method for assembling a double-deck vehicle body. The system includes a vehicle body chassis support and an upper floor support. The vehicle body chassis support includes a crossbeam, end supports, and an intermediate support, where the end supports are arranged separately at two ends of the crossbeam, the intermediate support is arranged at the middle of the crossbeam, a height of the end supports is greater than a height of the intermediate support, an upper surface of the end supports and an upper surface of the intermediate support are configured to be closely fitted to a lower surface of a vehicle body chassis; and the upper floor support is detachably arranged on a vehicle body chassis for supporting an upper floor of the vehicle body.