To enhance the complexity of fracture networks during hydraulic fracturing, degradable fibers and particulates are often injected into the pre-existing fractures as diverters to form temporarily plugging, initiate new fractures and make them deflect during extension. However, the plugging mechanism of fibers and particulates in hydraulic fracture is still not clear, which restricts the further improvement of the effect of the diverting hydraulic fracturing. Therefore, a visualized experimental system was built in this paper to observe the plugging process in a 5 mm wide hydraulic fracture. Through the experiments, it was found that the plugging process starts from the attachment of fibers on fracture faces, then the attached fibers keep gathering to form plugged zones, the plugging zones capture the flowing particulates to fill into the pores between fibers, accelerate the plugging process and plug the fracture eventually. The results indicated that the fibers could initiate the plugging process, since the plugging can be formed not only by the 2 mm-diameter particulates. The particulates could accelerate the following plugging process and withstand the pressure difference by acting as the skeleton of the plugged zones. It was suggested that fibers should be injected into the hydraulic fracture first and the particulates should to be injected into the hydraulic fracture when the pressure rises obviously. Finally, the concentration of fibers and particulates were fixed as 1% separately, and the plugging performance in a 5 mm wide fracture was investigated when the concentration of another component changes. It was found that the plugging forming time decreased with the increase of the diverters’ concentration. However, when the concentration of fiber or particulate is higher than 1.0%, the influence of the concentration on the plugging efficiency becomes very limited. Therefore, considering the plugging efficiency and material cost, it was recommended that the optimal mass concentration of particulates and fibers should be 1.0% for a width 5 mm hydraulic fracture.