In this work, 4-point bending fatigue performance including the fatigue damage evolution process, stiffness degradation process and fatigue lifetime of eco balsa cored composite sandwich structures have been investigated. An original triple dog-bone shape has been used to facilitate the observation of the induced laminate skin and core damages thanks to a localized bending or shear stress under 4-point bending loading. The microscopic images have been investigated to explain the fatigue damage evolution process in three stages of stiffness decrease curve during the whole fatigue process. The Glass-Fiber-Reinforced-Polymer (GFRP) skin damages are the predominant failure modes under the fatigue loading.