A novel temperature-resistant fluid loss reducer FL is obtained by free radical copolymerisation. Thermal gravimetric analysis demonstrates that the fluid loss reducer FL has a strong temperature-resistance capability and its degradation temperature is higher than 440°C. An experiment investigating the effect of FL on the comprehensive performance of oil well cement slurry shows that filter loss can be controlled within 50 ml when the dosage of FL is 1·0% of the total mass of cement and quartz powder under the ultra-high-temperature condition of 200°C (bottom-hole circulating temperature). Cement slurry with FL exhibits good comprehensive performance under high-temperature and ultra-high-temperature conditions. The working mechanisms of the fluid loss reducer FL are studied and the compressible ‘viscoelastic adsorbed layer’ perspective is proposed, which could explain the excellent ability to control the filter loss of cement slurry under high temperature.