•Discover competitive cracking behavior in accelerated CCF failure for turbine blades.•Find crack initiation change from slip planes to metallic pores and carbides, to oxides.•Reveal damage mechanisms in acceleration states by microstructural dominant features.•The source of cracking transition from transgranular to intergranular mode is the detected behavior.•Provide a promising mechanism insight for CCF estimation and acceleration test. The cracking behavior and microscopic mechanism of K403 superalloy turbine blade are investigated respecting the Combined high and low Cycle Fatigue (CCF) with four acceleration states. It concludes that: (1) the crack initiation sites transform from slip planes inside alloy matrix to subsurface pores and carbides, then to oxides outside surface with increasing loads; (2) the behavior in (1) is attributed to the competition and alliance of different microstructural factors and the interaction of the factors with grain boundaries; (3) hereinto, the role shift of high cycle fatigue in CCF causes the transformation of transgranular to intergranular cracking mode.