To improve the high-temperature resistance and density upper limit of environmentally friendly water-based drilling fluids, the fluid-loss reducer EHT-FL and the thinner EHT-TH were developed by modifying plant extracts of natural polyphenol. Two synthesized products were characterized by infrared spectroscopy. In laboratory experiments, the fluid-loss reducer EHT-FL and the thinner EHT-TH exhibited good high-temperature resistance. And the EC50 reached 2.38×105 mg/L and 4.49×105 mg/L, respectively, with no biological toxicity. The medium-pressure and high-temperature/high-pressure fluid-loss of freshwater-based mud with 2.0% EHT-FL are only 7.2 mL and 23.6 mL after aging at 200℃. When the dosage of EHT-TH is 1.0%, the apparent viscosity of high-density water-based drilling fluid (ρ=2.0 g/cm3) after aging at 200℃ decreases from 106.5 mPa·s to 66.5 mPa·s, and the high-temperature thickening phenomenon disappears. Based on the excellent temperature-resistance of EHT-FL and EHT-TH, a high-temperature and high-density (ρ=2.0 g/cm3) environmentally friendly water-based drilling fluid has been developed. This drilling fluid has reasonable rheological and fluid-loss properties. It has high-temperature resistance of 210℃ and resistance to 25% salt contamination, 1% calcium contamination, and 10% inferior soil contamination. EC50 of this drilling fluid is 4.19×104 mg/L, BOD5/CODCr is 29.23, heavy metal content is lower than the standard value, non-toxic, biodegradable. Therefore, this environmentally friendly drilling fluid can meet the drilling needs of deep-wells complex formations.