Relative permeability plays an essential role in evaluating reservoir characteristics in reservoir engineering. However, due to the tight reservoirs of low permeability and low porosity, conventional methods cannot accurately measure the relative permeability curves, even in a long time. The objective of this work is to develop an analytical model of spontaneous imbibition considering the influence of capillary pressure, which can be used to obtain the relative permeability curves for fight reservoirs. We derived an equation about fractional flow function (F(S-w)) and nonlinear coefficient (D(S-w)). Then combined with an empirical relative permeability model with uncertain parameters, a numerical method is used to calculate the fractional flow function, and an analytical water saturation curve can be drawn. After that, fitting CT scanning data with the analytical water saturation curves, a detailed relative permeability curves were acquired. The results show that relative permeability curves can be obtained by this method, and only use a spontaneous imbibition and CT scanning data, avoiding waste of time for testing relative permeability curves. The proposed model provides a more efficient tool for evaluating the relative permeability of the tight reservoirs.