Low cyclic fatigue testing of 45 medium carbon steel with the annular V-notch indicates that the propagation of crack under variable amplitude increase by degrees overloading is faster than that under constant amplitude overloading. Some problems which include the effect of constant amplitude overloading to propagation speed of crack, the suitable loading process of accelerating propagation of fatigue crack, and the fracture figure in suitable loading, should be discussed. Based on the experimental results and the analysis of the fracture surfaces of the specimens, the mechanism of propagation of fatigue crack under the overloading is explored. It is proposed that at the moment of the application of the overloading, the obvious instantaneous increase of crack growth rates is due to crack blunting. Then an adequate way of loading to make propagation of crack quickly is found that provide a basis for the engineering application of extra-low cycle fatigue fracture.