To address the inadequate mechanical strength of alginate-based mulch films, this study presents an innovative approach employing a dual network strategy. The dual network consists of the alginate network cross-linked by gradually released calcium ions from nanometer-sized calcium carbonate and the micronano multiscale fiber network formed by mechanical entanglement and hydrogen bonds between microfibrils. The multiscale fibers were fabricated through a mechanical process with no fiber loss under normal atmospheric pressure. Scanning electron microscopy, transmission electron microscopy, and X-ray diffraction were used to examine the microstructure and crystallinity of the multiscale fibers. The obtained dual-network mulch films showed good characteristics, with optimum mechanical strength, water vapor permeability, and biodegradability. Notably, the stress strength of the mulch film reinforced with the multiscale fibers was approximately ten times greater than the tensile strength of the control film, reaching 84.7 MPa when CFBC was 7.7%. The multiscale fibers present a novel approach for material preparation and reinforcement due to their high yield, low energy consumption, easy preparation process, and good compatibility with other polymers.