Reinforced concrete (RC) structures taking full advantages of concrete and reinforcing steel bars are widely applied in civil engineering. Concrete bridges are subjected to alternate loads as well as static loads, much importance should be attached to their fatigue. Reinforced concrete beams are the elementary members of concrete bridges. Fatigue failure mode and fatigue life prediction of normal or high-strength RC beams were the research focus at home. The fatigue behavior of low-strength RC beams was studied through four-point bending fatigue test in the paper. The test results indicated that all beams fractured for concrete shear failure, which made the fatigue life of low-strength concrete beams drop greatly compared to that of normal or high-strength concrete beams, because the fatigue failure of normal or high-strength concrete beams were caused by the fracture of one or more reinforcing steel bars; the mid-span deflection development of low-strength RC beams had three phases, and the middle phase occupied about 90% of whole fatigue life, also in the second phase the mid-span deflection developed linearly with the increasing of cycle numbers. This research work provides necessary basis for the fatigue life deterioration of low-strength RC beams.