Considering timevarying mesh stiffness and static transmission error(STE),a 6 degreeoffreedom dynamics model for a pair of spur gear was established, to reveal the effects of the fluctuations of driving and load torques on dynamic responses of gear pair.The numerical algorithm was adopted to solve governing equations to obtain dynamic transmission error (DTE) and linear vibration displacements, with the fluctuations incorporated. Furthermore,the influences of STE, rotating speed and relative phase angle of torques on DTE and the displacements were revealed. The simulation results indicate that peakvalley value of DTE increases with the increase of STE, rotating speed of pinion and the phase angle; while the peakvalley value is reduced to some extent when the fluctuation comes up with sinusoidal profile; the amplitudes are amplified with increasing of the fluctuation.