Cellulolytic bacteria are considered to be one of the most important parts in rumen. However, it is still lack of effective approaches to enrich or isolate those uncultured cellulolytic bacteria. The magnetic nanoparticle-mediated isolation is a new method to isolate or enrich the uncultured bacteria. This research aimed to isolate and enrich ruminal cellulolytic bacteria. The magnetic nanocomposites (MNP) were synthesized and coated with cellulose. Transmission electron microscopy, X-ray diffraction, and value stream mapping were applied to characterize the characterization of MNP. MNP coated with cellulose (MNPC) (17.4 mg/mL) was mixed with S. bovis(1.0 × 10⁸ cfu/mL) for 20 min to evaluate its capture efficiency, and for 60 h to evaluate the bacteria stability on MNPC. MNPC was mixed with Cellulomonas flavigena (C. flavigena, cellulolytic bacterium) and Streptococcus bovis (S. bovis, non-cellulolytic bacterium) for 60 h incubation to evaluate the isolation specificity of this approach. The bacterial in liquid were sucked out every 12 h and were determined by qPCR. To apply the method to enrich rumen cellulolytic bacteria, MNPC was mixed with rumen bacteria and incubated for 90 h. The bacteria in liquid were sucked out every 10 h and were determined by qPCR. Results showed MNP were globular and had average size of 20 nm and magnetization of 3.3–24.9 emu/g. The capture efficiency of MNPC was 99%. The stability of S. bovis on the MNPC was 90.24% after 60 h. The enriched C. flavigena raised from 6.49 × 10² cfu/mL to 2.20 × 10⁶ cfu/mL while S. bovis could not metabolize cellulose and captured by the MNPC with 1.05 × 10³ cfu/mL in the liquid. Relative abundance of the typed rumen cellulolytic bacteria, e.g., unclassified Butyrivibrio, unclassified Fibrobacter, and R. flavefaciens increased from 1.06%, 0.01%, 0.10% to 7.02%, 0.09%, 0.26%, respectively after 90 h incubation. The growth trend of a species from uncultured Lachnospiraceae (relative abundance of 28.33%) is related to that of the typed cellulolytic bacteria, so it was indicated that this species was a potential cellulolytic bacterium. This study provided a new approach to cultivate the uncultivable cellulolytic microorganisms.