A double-deck rail vehicle and a vehicle body thereof are provided. An underframe (11) of the vehicle body comprises a lower-layer underframe (111), end underframes (112, 115), sealing plates (114, 116), and side underframes (118, 119); wherein the lower-layer underframe is provided with an underframe middle beam (1111); the end underframe is fixedly connected to the underframe middle beam by means of reinforcement middle beams (113, 117); the sealing plate is fixedly connected between the end underframe and the lower-layer underframe; and the side underframe comprises side beams (1181, 1191) of the side underframe that are integrated, and the side underframe is fixedly connected to the end underframe by means of the side beams of the side underframe. The vehicle body supplements the reinforcement middle beams capable of increasing the connection strength between the end underframe and the lower-layer underframe, and uses the integrated side beams of the side underframe. Therefore, the structure strength, rigidity and compression-resistant performance of the vehicle body can be enhanced, so that the safety of the double-deck rail vehicle can be improved, and a problem that a related double-deck vehicle body does not satisfy the requirements of America strength standard can be solved.