To investigate the influence factors of the ultimate bearing capacities of Q420 steel U-rib stiffened plates subjected to axial compression, four model tests with different dimensions were conducted, and ultimate bearing capacities, instability modes and failure mechanism were discussed. The results showed that the instability modes of Q420 U-rib stiffened plates can be classified into three sorts, which are local buckling failure, global buckling failure and hybrid buckling failure. When the ratio of bending stiffness between U-rib and plate decreases, the instability mode changes from local buckling to global buckling or hybrid buckling. JRA underestimates the ultimate bearing capacity of U-rib stiffened plate, while AASHTO predicts more accurately.