Objective: Cross-linked guar gum fracturing fluid requires gel breaking and flowback after fracturing operations, but persulfate-based gel breaking agents face the challenge of low efficiency at low temperatures. Enzym gel breakers can achieve effective degradation of fracturing fluids under low-temperature conditions, serving as a necessary complement or substitute to chemical gel breakers. This study aims to screen highly efficient guar gum-degrading bacteria and subsequently optimize their enzyme production performance, thereby providing novel microbial resources and highly active enzymes for the development of enzymatic gel breakers. Methods: A guar gum-degrading bacterium has been isolated from fracturing flowback fluid and identified through morphological analysis and 16S rRNA gene sequencing. Its optimal medium formula and conditions were determined through single-factor experiments and response surface optimization. β-mannanase was purified through successive steps of ammonium sulfate salting-out and chromatographic techniques. The purified enzyme was then employed for testing its enzymatic activity and gel-breaking efficiency. Results: Based on phylogenetic analysis of the 16S rRNA gene sequence, the bacterial isolate was identified as Exiguobacterium acetylicum. To our knowledge, this study reports the first discovery of guar gum degradation capability in Exiguobacterium acetylicum. After optimizing the culture conditions, including carbon and nitrogen sources and shaking speed, the activity of β-mannanase and biomass of the guar gum-degrading strain increased by 198.82% and 103.09%, respectively. For crosslinked guar gum gel, the purified gel-breaking enzyme achieved a viscosity reduction rate of 92.3% to 95.7% at a low addition amount, 40°C, and pH 10, with a residue content of only 148 mg·L-1, which was 45.5% of that of ammonium persulfate gel-breaking. A complete gel break was achieved with the viscosity of the cross-linked guar gum gel dropping to less than 5 mPa·s. The residual debris from the ammonium persulfate breaking could be further degraded by the enzymatic breaker solution. Conclusion: This study successfully isolated a novel strain of guar gum-degrading bacterium and optimized its culture conditions. The enzyme produced by this strain exhibits excellent low-temperature gel-breaking performance, enabling highly efficient degradation of cross-linked guar gum fracturing fluids at low temperatures with minimal residue.