The invention discloses a method for determining relative permeability of rocks. The method comprises the following steps: (1) preparing rock samples with different water saturation; (2) applying confining pressure to the rock samples to a preset value, applying gas permeation pressure after the confining pressure is stable, then maintaining the permeation pressure constant and loading the confining pressure step by step, and measuring and recording the gas effective permeability of the rock samples with different water saturation under specific permeation pressure and different confining pressure; (3) calculating the gas measuring relative permeability of the rock samples with different water saturation under different confining pressure, drawing the curve relationship between the water saturation and the relative permeability, and obtaining the function relationship between the water saturation and the relative permeability through curve fitting, so as to determine the relative permeability of the rocks under specific water saturation. The effective permeability under different confining pressure is measured by loading the confining pressure step by step, so that the relative permeability is calculated, and the functional relationship between the water saturation of the rock samples and the relative permeability is constructed, and the relative permeability of the rocks can be determined conveniently and accurately.