To address the insufficient temperature resistance of key additives in oil-based drilling fluids, a high-temperature resistant modified oxidized asphalt filtrate reducer was prepared using asphalt as the main raw material. The structure of the modified product was characterized by FTIR and SEM. The filtrate reduction effect of the modified asphalt was evaluated under different dosages and temperatures. The results showed that with an increase in modified asphalt dosage, the HTHP filtrate loss decreased from 5.6 mL to 2.7 mL. Although the HTHP filtrate loss increased from 3.8 mL to 5.6 mL with an increasing temperature at the same concentration, the growth rate under high-temperature conditions was only 12%, demonstrating excellent filtrate loss reduction performance.