This study systematically investigated the fatigue property of DD6 nickel-based single crystal superalloy using in situ scanning electron microscopy (SEM). A pioneering exploration was conducted on the coupled effects of secondary orientations ([010] and [110]) and recrystallization defects on fatigue performance, representing the first comprehensive report of this synergistic interaction. Surface recrystallization was found to significantly deteriorate fatigue resistance, with preferential crack nucleation observed at recrystallized regions. Orientation-dependent performance analysis revealed the superior fatigue durability of [110]-oriented specimens over [010]-oriented counterparts in both pristine and recrystallized conditions. Distinct slip mechanisms were identified: [010]-oriented specimens exhibited pronounced cross-slip activity, while [110]-oriented samples maintained single-slip deformation characteristics. Fatigue crack propagation in [010] orientation was governed by multi-octahedral slip system activation. Combined electron backscatter diffraction (EBSD) analysis and crystal plasticity finite element modeling (CPFEM) were employed to elucidate orientation-dependent plastic deformation mechanisms. CPFEM simulations demonstrated that [010]-oriented specimens accumulated higher cumulative shear strain (CSS) under equivalent stress condition compared to [110]-oriented samples, directly correlating with reduced fatigue life. A unified fatigue life prediction model was developed incorporating both secondary orientation effects and recrystallization defects, establishing a robust theoretical framework for evaluation of DD6 alloy under cyclic loading conditions.