Abstract: Proper fracturing fluid system is critical to successful deep well (more than 5000m deep) fracturing with abnormal high temperature (higher than 180 ℃). However, conventional fracturing fluid systems now used in China only adapt to the strata with temperature lower than 150 ℃. Lab experiment is conducted to modify two major additives that affect the property of ultra-high temperature fracturing fluid---thickener and crosslinker. By strictly controlling the reaction conditions, GHPG gum/borate gelling thickner with low residue and high viscosity for ultra-high temperature use and BA1-21 delayed crosslinker for ultra-high temperature use are obtained, and a fracturing fluid system resistant to the temperature higher than 190 ℃ is developed successfully. The performance of modified gum/borate gell for ultra-high temperature use is evaluated, the delayed crosslinking performance and heat/shearing resistance of crosslinker of the fracturing fluid system for ultra-high temperature use is also evaluated. Eight days after fracturing and modification in ZG63 well, flow back rate of fracturing fluid attains to 74.9%, with the average being 6000 m3. The reservoir is modified and recognized. This new breakthrough is valuable for deep reservoir fracturing with ultra-high temperature in China.