Cement-modified red mud as roadbed filler was investigated through tests, including compaction, California Bearing Ratio (CBR), modulus of resilience (E0), unconfined compressive strength (UCS), and scanning electron microscopy. Optimum moisture content (ω0), maximum dry density(), CBR, modulus of resilience, UCS, and microscopic characteristics of red mud were analyzed in different modified states. An optimal cement admixture for red mud modification was proposed. Results showed that (1) adding cement significantly improved CBR and modulus of resilience, (2) cement-modified red mud exhibited faster initial strength formation, with 7-day UCS over 65% of 28-day UCS, (3) water stability and frost resistance improved significantly with 4% and 6% cement dosing, (4) recommended cement admixture was 1.1% without adverse conditions, but not less than 6% with water immersion. Modified red mud is unsuitable for seasonal frozen regions. The recommended blending and leaching concentration met regulatory requirements, meeting technical, environmental, and economic goals for roadbed filler in highway construction.