The 8Cr4Mo4V steel is the key material used in aero-engine main shaft bearings. However, the mechanism its surface roughness controls tribological properties under coupled multi-physics service conditions remains unclear, which restricts the long-life design of aerospace bearings. In this study, 8Cr4Mo4V steel specimens with controlled surface roughness (Ra = 0.02, 0.04, and 0.06 μm) were prepared, and ball-on-disk tribological tests were conducted under variable loads, sliding speeds and ambient temperatures to clarify the roughness-dependent friction and wear behavior. The results show that the effect of surface roughness on the tribological properties depends strongly on working condition: it was the most prominent under low load (8 N) and low speed (0.1 m/s), where the coefficient of friction (COF) increased by 22.95% as roughness rose from 0.02 to 0.06 μm, while the effect was significantly weakened or even completely masked under medium–high and high sliding speeds. The ambient temperature fundamentally altered the regulation mechanism of roughness: at 600 ℃, an abnormal maximum wear rate (WR) occurred at the critical roughness of 0.04 μm, which was 2.17 times that of the 0.02 μm sample, breaking the traditional positive correlation between roughness and wear. The core mechanism is the dominant effect of roughness by asperity contact at room temperature, while the dynamic formation-rupture equilibrium of interfacial oxide film at elevated temperatures governs. This work establishes the matching design criterion between surface roughness and service conditions for aerospace bearing steel, providing theoretical basis and experimental support for surface finishing optimization of aero-engine bearings.