•The increase in load-bearing capacity of the FCF is dependent on the strength increase of the filling body.•The filling body restrains the lateral deformation of the coal pillar to increase its load-bearing capacity.•The load-bearing process of FCF has a distinct stage of transition between the primary and secondary load-bearing structures.•As the width of the coal pillar increases, the crack initiation gradually moves from the pillar to the filling body. The continuous mining and filling cemented coal mining (CMCB) technology solves the problem of unbalanced mining and filling efficiency in the coal mining with lane-type cemented filling. In order to study the alternate bearing characteristics of the roof of the field under this process, it is divided into the coal pillar bearing area, co-bearing area, and the filling body bearing area. In order to further study the mechanical properties and failure characteristics of the composite load-bearing structure composed of the filling body - coal pillar - filling body (FCF) in the co-bearing area, the strength growth law of the FCF and its crack expansion law is revealed through the uniaxial compression test. And the primary and secondary load-bearing relationships between the coal pillar and the filling body were obtained by the digital image correlation (DIC) method and the static strain test. The influence of coal pillar width on the strength of the FCF is further revealed by establishing the PFC model. The results show that: increasing the strength of the filling body can improve the overall strength of the FCF both directly and indirectly; while the interaction between the filling body and the coal pillar is distributed as large in the middle and small at the ends, and increases significantly with the increase in the strength of the filling body; the cracks of the FCF appear first at the weak structural surface formed by the coal pillar and the filling body, and the coal pillar is the main load-bearing structure before the peak stress, and gradually transitions to the secondary load-bearing structure in the post-peak deformation stage. The test results can be used as a reference for the design of the width of the lane, the optimization of the mining process, and the proportioning of the filling slurry.