Abstract: To effectively alleviate the damage of residues produced by broken guar gum fracturing fluid to the formation, the gel breaking mechanism of biological enzyme and ammonium persulfate/biological enzyme composite gel breakers on the guar gum fracturing fluid was investigated by analyzing the molecular physical and chemical properties of the gel breaking solution of guar gum fracturing fluid under different gel breaking methods. The results show that compared with ammonium persulfate gel breakers, biological enzyme and ammonium persulfate/biological enzyme composite gel breakers can effectively reduce the relative molecular weight and molecular size of gel breaking solution. The degradation products in the gel breaking solution are mainly disaccharide to pentasaccharide. The analysis of gel breaking solution residues shows that the content ratio of mannose to galactose in residue molecules is only 0.38, which is the main reason for its poor water solubility and the existence of residues in the gel breaking solution. In addition, through the simulation test, it is found that acid injection can effectively degrade the gel breaking solution residues of guar gum fracturing fluid and improve the proppant conductivity. The results can provide a theoretical basis for selecting gel breakers and reducing damage caused by gel breaking solution residues.