Abstract: The application of guar gum in oilfield exploitation, especially in fracturing fluids, has gained increasing attention. As it is demanded, the guar gum in fracturing fluid must be subsequently degraded after being injected into the fracture produced in the process of well stimulation, and thus enable the oil to flow to the wellbore. For this purpose, the fast and effective degraders of guar gum are anticipated, and the apparent viscosity and degradation products are considered to be very important factors that influence the effect of hydraulic fracturing. Mannanase is an effective degrader of guar gum and shows great potential in oil reservoir fracturing due to its high activity and environment friendly nature. However, till now, the knowledge on the performance of mannanase to degrade guar gum under extreme temperature and pH conditions is limited. In this study, mannanase-catalyzed guar gum degradation at different temperatures and pH values was analyzed mainly by combination of GPC and ESI-MS. Comprehensive data with respect to guar gum degradation were obtained including the composition, molecular weight distribution, and reducing sugar content of degradation products as well as the reduction of viscosity of guar gum solution after degradation. The results revealed that mannanase Man-14 showed high guar gum degradation activity under a wide range of temperature and pH. The viscosity of guar gum solution was reduced to a value below 5 mPa·s in 20 min by 0.49 U Man-14 at the temperature between 55℃ to 70℃ and pH between 7.0 to 9.5. Furthermore, the molecular mass of the degradation products produced by Man-14 were much smaller than those by APS even with the same post-degradation viscosity of guar gum solution. The molecular mass of guar was reduced by enzymatic degradation from the initial 2×10⁶ Du to 3 800 Du within 3 h at the temperature 35℃ or 70℃, and the oligosaccharides with polymerization degree of 2-5 was detected by ESI-MS analysis. This study provides more information on guar gum degradation with mannanase and is helpful for its applications, especially in oil reservoir fracturing for breaking guar gum.