In recent years, the misoperation of gas protection caused by the external short-circuit impact occurs frequently, which threaten the operation efficiency of the transformer. Under the background, in order to explore oil flow characteristics in the pipeline when the external short-circuit fault occurs, this paper establishes a transient oil flow measurement system on the power transformer and carries out an external short-circuit fault test. The test results show that the transformer winding deformation and vibration caused by the short-circuit current will lead the oil flow through the pipeline. Due to the inertia of the liquid, the time delay of oil flow measured in this paper is up to hundreds of milliseconds. In the actual operation, the time of gas protection misoperation lags behind the external fault for hundreds of milliseconds, which keeps consistent with the delay phenomenon of oil flow action. The test also shows that the oil flow surge is affected by the severity and location of the fault. In addition, the results of the oil flow action delay also provide an essential reference for further study on the mechanism of oil flow shock and the misoperation of gas protection.