Abstract: The evolution of plastic deformation at the crack tip during fatigue crack propagation of TC4ELI alloy under the same load amplitude and high and low load ratios (0.7,0.1) was investigated by digital image correlation (DIC) method . The results show that the ratchet strain of the alloy and the degree of monotonic plastic deformation at the crack tip under 0.7 high load ratio were larger, and the strain accumulation was faster. The equivalent strain amplitude under 0.1 low load ratio was larger and the cyclic plastic deformation was more obvious. The ratchet strain and the gradient of equivalent strain amplitude under the high load ratio were both larger than those under the low load ratio, and the strain at the crack tip was more concentrated. The size of the plastic zone at the crack tip had a power function relationship with the stress intensity factor. Under the high load ratio, the index between the size of the monotonic plastic zone and the maximum stress intensity factor was relatively large, indicating that monotonic plastic deformation played a dominant role in crack propagation the dominant role was strengthened with the increase of crack length or crack propagation rate. Under the low load ratio, the index between the size of the cyclic plastic zone and the amplitude of the stress intensity factor was relatively large,indicating that cyclic plastic deformation played a dominant role in crack propagation; the dominant role was weakened with the increase of crack length or crack propagation rate.