The invention discloses a U-shaped auxiliary frame stiffness testing device, and belongs to the technical field of railway freight car parts detection. The U-shaped auxiliary frame stiffness testing device comprises a left positioning rod, a right positioning rod, a sliding beam connecting the two rods, a positioning seat, and a dial indicator. The U-shaped auxiliary frame stiffness testing device further comprises an L-shaped mounting seat. The transverse edge of the L-shaped mounting seat is provided with a clamping groove in which the dial indicator is placed. The longitudinal edge of the L-shaped mounting seat is connected with the sliding beam. One end of the positioning seat is provided with a U-shaped opening, and the other end is provided with an end plate. An ejector rod of the dial indicator is in contact with the end plate. The U-shaped auxiliary frame stiffness testing device of the invention has a simple and compact structure, is quick and firm in installation, reliable and durable, and convenient to manufacture, and can ensure the accuracy and reliability of test. The test cost is low, the versatility is high, and operation is convenient and flexible.