The invention discloses a universal cross beam mechanical arm welding clamp for urban rail vehicles. The universal cross beam mechanical arm welding clamp for the urban rail vehicles comprises a base. A control switch is installed in the middle of the front side face of the base. The input end of the control switch is connected with the output end of an outer power source. The upper surface of the base is provided with four installation frames. The front two installation frames are fixedly connected through a connecting arm, and the back two installation frames are fixedly connected through a connecting arm. The upper end of each installation frame is fixedly provided with a fixing frame. The universal cross beam mechanical arm welding clamp for the urban rail vehicles is accurate in locating, a side frame is fixed through cooperation between locating grooves in locating plates and the fixing frames, a cross beam is fixed through a cross beam frame, the adjusting capacity is high, the interval between the front fixing frames and the back fixing frames can be adjusted, the cross beam is made to be in tight contact with the side frame, a side plate is driven by a second drive air cylinder to conduct aligning operation on the side frame, and the welding accuracy of a bogie is guaranteed; and the universal cross beam mechanical arm welding clamp for the urban rail vehicles is suitable for multiple types of vehicles, avoids time waste caused by vehicle type switch and is suitable for welding of the cross beams of the bogies.