Abstract: To reduce operation cost, sea water is frequently used to formulate fracturing fluids for fracturing offshore wells. Sea water generally has higher hardness and higher salinity, which are adverse to the viscosity development of fracturing fluids. To solve this problem, a guar gum was first physically modified by molecule reconstruction followed by chemical modification with hydroxypropylation. This physically and chemically modified guar gum of a degree of substitution (D.S.) of 0.45 can efficiently viscosify sea water, with rate of viscosification reaching at 90% in 3 min. Compared with conventional hydroxypropyl guar gum of the same D.S., the molecule-reconstructed hydroxypropyl guar gum viscosifies seawater in an obviously fast manner. By testing the performance of different crosslinking agents, the dendrimer organoboron crosslinking agent was selected because the diffusion rate of boron atoms in the course of crosslinking is low, by using scale inhibitor, the crosslinking process can be delayed, and a gel whose average microgrid size of 40-50 nm was formed. The grid size is greater than the grid size of a gel formed by low molecular weight complex agent instead of dendrimers, and this helps delay the crosslinking rate of guar gum effectively and thus reduces the friction and drag of the fracturing fluid. Laboratory evaluation of the seawater fracturing fluid formulated with molecule-reconstructed hydroxypropyl guar gum and dendrimer organoboron crosslinking agent showed that after aging at 120℃ for 2 hours, the viscosity of the fracturing fluid was 200 mPa.s, greater than the value specified in the industrial standard and the flow resistance was reduced by 80%. The viscoelasticity of this fracturing fluid is greater than the boron-complexed guar gum using low molecular weight complex agents.