A method for calculating the oil flow rate in the normal switching process of an on-load tap-changer for a transformer divides the calculation process into two stages, namely, the calculation of the oil flow rate of a flow field in the gas expansion process under the action of an electric arc and the calculation of the oil flow rate of the flow field in the two-phase flow process after the electric arc is extinguished. The method comprises the following steps: firstly, calculating arc energy according to switching current of an on-load tap-changer, and calculating gas generation speed and bubble final volume in a switching process by utilizing a relation between the arc energy and gas generation; Secondly, setting the gas generation speed as an initial condition, setting a slip-boundary-free condition according to the specific structure of the on-load tap-changer, and listing and writing a flow field equation to calculate the oil flow rate; Secondly, setting the initial state of the two-phase flow according to the arc generation position and the gas generation volume, solving the phase interface change by using a level set method, and coupling the phase interface change with the flow field basic equation to form a two-phase flow process flow field equation to calculate the oil flow rate in the two-phase flow process; The method provided by the invention can avoid misoperation of gas protection of the on-load switch due to oil flow surge in the normal switching process.