In order to improve the thermo-stability of 2-acrylamido-2-methyl-propane sulfonic acid （AMPS） copolymer and enhance the performance of cement slurry at high temperature，a fluid loss additive，shorted as GP，was synthesized by grafting mixed monomers of AMPS，acrylamide（AM），N， N-dimethylacrylamide（DMAM）and acrylic acid（AA）onto humic acid. The structure of GP was characterized with FT-IR spectroscopy，gel permeation chromatography（GPC）as well as thermogravimetric analysis（TGA） . The effect of temperature on the viscosity of GP solution was studied，and its performance in cement slurry was evaluated at lab scale. The research results showed that the GP was a graft copolymer with a thermal-decomposition temperature of 282℃. Although the viscosity of GP aqueous solution decreased with increasing temperature，its relative viscosities at different temperatures were higher than that of linear polymer LP solution. Compared with their initial values at room temperature，the viscosity retention of GP solution at 90℃ was 66.3%，which was 17.6% higher than that of LP solution. GP had excellent fluid loss control ability and salt-tolerance. The viscosity of cement slurry containing GP（CSG）at room temperature was lower than that of cement slurry containing LP. Moreover，when CSG was heated from 30 to 150℃，its viscosity retention maintained 60.7%，which was 15.8% higher than that of LP cement slurry. GP also exhibited well compatibility with other additives and ignorable effect on the thickening time and compressive strength. The compressive strength of CSG after setting at 50℃ for 48 h was 22.7 MPa，similar to that of LP cement slurry. Ultrasonic experiment revealed that the compressive strength of CSG developed normally. The transition time of static gel strength was 17% shorter than that of LP system，which was benefit to prevent the gas migration during solidifying period of cement slurry.