The relative permeability is important for researching seepage fluid mechanics and driving characteristics. The emphasis of this review is methods to calculate relative permeability curves by using the relationship between capillary pressure and saturation. The Pressure-Saturation relationship and fractal theory is considered as a key technology to push the development of the methods. According to the introduction of the fractal theory, the methods were divided into two categories. The principle to calculate relative permeability curve and the principle of fractal theory were described. The research history and current situation of methods to calculate relative permeability by using capillary pressure were introduced, which includes the main outcomes of the researchers. Finally, three main directions of the capillary pressure had been pointed out: 1) determination of the relative permeability curves to unconventional reservoirs; 2) the determination of relative permeability curves to special fluids; 3) the combining application with other core test methods.