The traditional water-lubricated bearings are difficult to form effective lubricating film under extreme low-speed and heavy-duty conditions, resulting in the low-load, high-resistance, high-vibration and high-noise shortcomings. The deep-physical mechanisms are not clear. The paper investigates the mixed lubrication performances and critical characteristics for water-lubricated bearings under extreme conditions. A novel multi-field coupling lubrication model is proposed. The functional mapping relationships among lubrication performances, the critical characteristics and the influencing factors are developed. The rotor dynamic models considering surface micro-topography are built. The variation mechanisms of the dynamic behaviors are revealed. The theoretical analysis is consistent with experiment, which proves the model and algorithm's correctness. The academic value of the paper is expected to achieve a novel breakthrough in formation of effective hydrodynamic film under extreme severe conditions for such bearings.