For plugging lost circulation in high temperature formations, normal gels with lower thermal stability are unable to be applied. To solve the problem, a novel two-step crosslinking gel containing thermal stable polymer GN-1 was developed, which performed high temperature resistance. The optimized gel system is fresh water 1.5%GN-1 0.15% ground crosslinkers 1% downhole crosslinkers 0.2% antioxidant. Thermal stability of the gel was evaluated via hot rolling and high temperature rheological measurement. After hot rolling at 180℃ for 24h, the shear viscosity of the gel at the rate of 7.34s-1 was above 10000mPa?s. The viscoelasticity of the high-density gel system remained good after aging for 24h under 180℃. The results showed that the gel had excellent heat-resisting property. The influence of environmental temperature and pH value to gelling performance and the compatibility with polysulfonate drilling fluid system was also discussed, which was very helpful to operation in future.