According to transformer vibrating mechanism, time-varying electromagnetic field and structural mechanics equations, the paper use COMSOL to establish a physical model of a three-phase transformer under the conditions of with load and without load, to make analysis of the magnetic flux distribution and the stress and strain in core and windings. The results vividly verify the following conlusions are available: Magnetostriction is the main cause of vibration under no-load and light load conditions. While we can’t ignore the contribution to transformer’s vibrations when they are full load. This does provide reference of how to decrease vibration and noise in the future.