The present disclosure relates to a method for determining oil and gas relative permeabilities of a reservoir. The method includes: drilling a cylindrical core from a to-be-tested reservoir; injecting an oil and gas mixture into the cylindrical core through the piston container at a set flow rate by using the relative permeability testing device, and reducing pressures at a control end of the back pressure valve in sequence to obtain experimental values acquired under each pressure at the control end of the back pressure valve; equivalently simulating developing performance of a producing well of the to-be-tested reservoir using a one-dimensional numerical simulation grid model based on current oil and gas phase relative permeability curves, so as to obtain simulation values at different times; constructing an objective function according to the experimental and simulation values; determining whether a value of the objective function reaches a preset target value.