The invention relates to a pore network model-based calculation method of three-phase relative permeability. Based on the construction of a pore network model, an underground rock pore structure is reproduced, and under the condition of coexisting of oil-gas-water three phases inside the rock core, flow of fluids is simulated to obtain three-phase relative permeability. By the utilization of an orthogonal design method during the process, design parameters which fit the best with a two-phase relative permeability curve obtained by an experimental method are selected so as to simulate three-phase flowing based on the pore network model. According to the invention, fluid physical properties and rock structure parameters are fully considered, and the physical process of fluids flowing in a porous medium can be well reproduced. Two-phase flow is very easy to realize by a physical experimental method, and the constructed pore network model is greatly screened during the process of matching results of the two-phase flow experiment.