Abstract <p><i>Fibrobacter succinogenes</i>, a Gram-negative bacterium in the phylum Fibrobacteres, has the ability to efficiently degrade cellulose by employing a new mechanism. Genomic analysis suggested <i>F. succinogenes</i> lacks exoglucanases and most of its cellulases lack recognizable carbohydrate-binding modules (CBMs). To explore the specificity of cellulase, Fisuc_1219, a typical GH8 endoglucanase, was expressed and characterized. Bioinformatics results showed that this enzyme possessed the (a/a)6 barrel that consists of 6 pairs of antiparallel a-helices. Fisuc_1219 was expressed in <i>Escherichia coli</i> and exhibited optimal activity at 40°C and pH 6.0 using carboxymethyl cellulose (CMC) as the substrate. This enzyme demonstrated stability under alkaline conditions, retaining approximately 80% activity at pH 8.0 and 9.0, and 50% activity at pH 10.0. It also displayed significant thermostability, maintaining 54% activity after 32 h at 70°C and 50% activity after 4 h at 80°C. The degradation products of CMC and regenerated amorphous cellulose included cellohexaose, cellopentaose, cellotetraose, cellotriose, cellobiose, and glucose, with cellotriose being identified as the predominant hydrolysis product. Among various metal ions, Mg²⁺ exhibited the strongest inhibitory effect on Fisuc_1219 activity, while chemical additives also showed varying degrees of inhibition. The characterization of Fisuc_1219 will provide theoretical guidance for clarifying the mechanism of cellulose degradation of <i>F.&#xa0;succinogenes</i>.</p>