There was swelling problem of thickening agent when the fracturing fluids prepared with seawater was continuously mixed during the offshore hydraulic fracturing operation. So the research on the swelling performance of guar gum was conducted. The effect of the type of modified group, the degree of molar substitutions, stirring speed, guar gum concentration, pH value and temperature on swelling performance of guar gum was studied. The results showed that the seawater sample was rich in⁺、Mg²⁺、Ca²⁺ and Cl^-、SO₄^2- with 34440 mg/L salinity. Under conventional conditions such as 20℃, 300 r/min and 7.5 pH value, the swelling time of carboxymethyl guar gum, hydroxypropyl guar gum (DS=0.3), hydroxypropyl guar gum (DS=0.15) and unmodified guar gum was 25, 50, 55 and 60 min, and the corresponding apparent viscosity of that was 63, 59, 45 and 23 mPa·s. The effect of other factors on swelling performance of carboxymethyl guar gum was studied and the optimum preparation condition was obtained as follows: 400—500 r/min stirring speed, 30—40℃ temperature, 6—7 pH value and dosage of guar gum within 0.5%. The swelling time could be optimized within 10 min to meet the requirement of continuously preparing seawater-based fracturing fluids.