Mineral fillers are widely used in paper industry, which can deliver satisfactory paper optical properties and production cost saving. However, its content in paper is limited due to the weak interaction between filler and cellulosic fibers. In recent years, cellulose nanofibrils (CNFs) has shown its potential prospects in reinforcement of composite materials and cellulosic fiber materials because of its high mechanical strength, large specific surface area and biodegradability. Herein, we reported a concept of flexible composite filler prepared by precipitated calcium carbonate (PCC) and cellulose nanofibrils through co-refining and syringe formation. The network encapsulation structure PCC/CNF composite was formed by intense co-refining mechanical force and the composite gel of CNFs and PCC was utilized to prepare fibrous filler. Fourier infrared spectroscopy (FT-IR), X-ray diffraction (XRD) were used to investigate the mechanochemical effects between CNFs and PCC during co-refining process. Morphology of the fibrous composite filler was assessed by scanning electron microscope (SEM) and 3D super depth of digital microscope. Paper physical test showed that the bulk and tensile strength of fibrous composite filler paper was increased by 5.43% and 42.58% than that of paper filled with PCC/CNF mixture at 60% filler content, while the optical properties of the paper was maintained well. Therefore, PCC/CNF composite filler exhibited the advantages in facile manufacture, excellent reinforcing property, which is favorable in highly filled paper. This fibrous flexible composite filler may provide a novel strategy for the design of papermaking filler, cellulose nanofibrils efficient utilization and manufacturing highly filled paper products.