<p>Spontaneous imbibition plays a significant role in numerous engineering problems, particularly in the context of oil and gas reservoir exploitation over several decades. To date, various models have been proposed to simulate this process. However, none of the existing models simultaneously consider the effects of variable cross-section, fluid viscosity, and gravity. This study assumes that capillaries exhibit tortuous, non-circular, and irregular axial variations. By fully considering the effects of fluid viscosity and gravity on imbibition behavior, it proposes a more generalized spontaneous imbibition model. The results calculated by the new model are compared with both published data and the numerical simulations conducted in this study, validating its predictive capability. The findings indicate that for symmetric C-D and D-C capillaries, the total imbibition time is independent of the arrangement order of capillaries with different diameters. In contrast, for asymmetric capillaries, the total imbibition time decreases as the number of pore-throat structures increases. When the number of pore-throat structures is large, the interface displacement is proportional to the square root of time (<i>t</i>^1/2), exhibiting behavior similar to that observed in uniform capillaries. Based on this observation, an expression for the equivalent diameter is derived. Additionally, the imbibition velocity in non-uniform capillaries is significantly lower than that in uniform ones, with greater differences in pore-throat lengths leading to shorter total imbibition times. Under consistent wetting phase viscosity, a higher non-wetting phase results in longer imbibition times.</p>