The mechanism of the effects of external short circuit current on oil flow inside the transformer is analyzed, and a SFPZ11-120 MVA/220 kV transformer transient electromagnetic-force-flow field coupled model is built in ANSYS Workbench. The vibration of windings caused by short circuit current is studied, and the effect of external fault on transformer oil surge is simulated accurately by finite-element method, from which the distribution of oil flow under external faults can be achieved. The simulation results show that the transformer oil surge caused by external short circuit current has great effect to gas protection, which may cause gas protection maloperation.The simulation models and methods can be generalized to oil transformers of other voltage level and can provide reference to analysis of transformer oil surge.