The invention provides a car underframe assembling system and method. The system comprises an underframe assembling frame and further comprises two end floor supporting devices which are arranged at the two ends of the underframe assembling frame, are fixedly connected with the underframe assembling frame and are used for supporting end floors of a ship-shaped underframe and a side beam supporting device which is fixedly connected with the underframe assembling frame and are used for supporting side beams of the ship-shaped underframe; the distance between the two end floor supporting devices is larger than or equal to the length of a middle floor of the ship-shaped underframe. According to the car underframe assembling system and method, the assembling requirement of the ship-shaped underframe is met, and the assembling efficiency and the assembling quality are effectively improved.