Abstract: The experimental research on fatigue surface crack propagation of thick and large specimen of 10Ni5CrMoV steel under biaxial-bending stress was carried out using the biaxial bending fatigue test method when the testing environment was similar to the real service one. Besides, the artificial crack source notch was introduced into the surface of the circular plate to simulate the surface crack defects existing in the actual component. It was indicated that the crack half length c and the crack depth a increased with the number of cycles N monotonically, and the curve of a-N presented a certain linear relationship; the prediction model between the the surface crack half-length c and the depth a was established, so that the problem that the depth of crack couldn't be measured directly could be solved quantitatively.