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The magnetic properties of the magnetic transparent composite films largely depend on the content of Fe3O4 employed. However, higher addition of Fe3O4 into the matrix will lead to Fe3O4 nanoparticles agglomeration resulting in the reduction in the transparency of the composite films. The contradiction between Fe3O4 nanoparticle content and transparency limits the application of transparent magnetic films. In this work, the spherical bamboo nanocellulose (SBN) with an average size of 61 nm was prepared from bamboo powder by NaOH/Urea/Thiourea aqueous system, which was an effective, inexpensive and less toxic strategy. The basic structure of SBN was investigated. The SBN exhibited the cellulose II type crystal structure. The molecular chain length of SBN were lower than before dissolution. The magnetic SBN/Fe3O4 composite films with light penetration were prepared. The results revealed that the dispersion of Fe3O4 nanoparticles in the SBN/Fe3O4 composite films was much better than that in nanofibrillated cellulose (NFC)/Fe3O4 composite films because of the abundant hydrogen bonds between Fe3O4 nanoparticles and SBN. The transparent results indicated the highest transmittance of the films was 94% in the wavelength range of visible light, which was almost 23.7% higher than that of NFC/Fe3O4 composite films with the same amount of Fe3O4 addition (15 wt%). The findings in this study also revealed that the SBN in the SBN/Fe3O4 composite films was distributed in layers. The saturation magnetization of the SBN/Fe3O4 composite films was 10.65 emu g−1 with 15 wt% Fe3O4 incorporated, which guaranteed the excellent magnetic properties.