The working principle of hydrodynamic journal bearing, and the features of oil film that are different from that of general mechanical elements were introduced. The relation between the load and the dynamic characteristics of bearing was analyzed. By modeling rotor-bearing system and deducing formulas, the conclusion that light load will cause weak oil-film stiffness which leads to larger vibration amplitude and oil whirl at the severe condition was put forward. The conclusion is successfully applied in dealing with typical fault of oil whirl and insufficient oil-film stiffness.