Phenolic resin is modified to improve its defect of poor acid resistance in high temperature environment. Under the condition of alkaline catalyst, furfural, phenol and formaldehyde were subjected to co-polycondensation reaction to produce thermosetting furfural-phenol-formaldehyde co-polymerization resin (FPF). P-toluenesulfonic acid was selected as the curing agent for curing the synthesized FPF, and the FPF curing compounds were prepared. The effects of synthesis conditions on FPF viscosity, solid content, and high-temperature and strong-acid resistance of FPF cured products were investigated. The results showed that when the reaction temperature was 95 ℃, the amount of catalyst was 6% of the mass of phenol, and the replacement rate of furfural to formaldehyde was 15%, the viscosity of FPF was 1 200 mPa∙s, and the solid content was 87.7%. Thermal weight loss test of FPF cured material showed that its maximum decomposition rate was 2.883%/min in the rapid decomposition stage, which was 33.1% lower than the decomposition rate of conventional phenolic resin (PF). The mass retention of FPF cures treated with high temperature and strong acid environment was 96.56%, which was 6.44% higher compared to the mass retention of PF cures. Furfural as a substitute for part of the formaldehyde to modify phenolic resins has improved acid resistance and thermal stability, and the modified phenolic resins can be applied in environments with strong acids and high temperatures.