The invention relates to a moment rigidity testing device and method for wheel hub bearing units. The testing device comprises a pedestal platform, a non-rotating member switching tool, a bearing outer ring, a flange, a bearing pre-tightening assembly, a rotating member switching tool, a symmetric I beam, a sliding block, four displacement sensors and an axial loading module, wherein the symmetric I beam is connected and fastened with the rotating member switching tool through bolts, the sliding block sleeves the symmetric I beam, a hemisphere surface is arranged on the upper surface of the sliding block to place steel balls, the axial loading module is arranged above the steel balls, and the four displacement sensors are respectively arranged on the surfaces of the flange and the non-rotating member switching tool. The moment rigidity of a wheel hub bearing unit can be more accurately and reliably tested, loading conditions of the wheel hub bearing unit can be simulated, and test error of small load moment rigidity can be lowered greatly. The repeated utilization rate of test tool material on products of different model is increased.