A single span rotor system with two discs was studied in this paper. Aiming at the oil-film instability problem of the sliding bearing at a high speed, the lumped mass model of the rotor system considered with the gyroscopic coupling effect was established. The left and the right bearing were simulated by the spring-damping model and the no-linear oil-film force model of the short bearing, respectively. The influence of eccentricity of rotor, bearing clearance, lubricating oil viscosity and slenderness ratio of bearing on the system vibration is presented under two conditions (conditions 1 and 2 denoting the eccentricity directions of two discs same and reverse, respectively ) at given rotating speeds. The results show that the first order oil-film instability appears under condition 1, and the first and second order oil-film instability all appears under condition 2. The system amplitudes under two conditions are almost consistent at a low rotating speed (3200 r/min); the amplitude under condition 2 is greater than that under condition 1 at higher rotating speeds (5000 r/min and 10000 r/min); some catastrophe points of the amplitude curves appear under the two conditions at a high rotating speed (13000 r/min).