This study applies a transparent micro model and digital image analysis to the experimental study of the displacement mechanisms for water and air in porous media during imbibition process, and examines the displacement formulas. This study conducts experiments following Lenormand's assumptions as closely as possible. Various displacement mechanisms were observed, and their images were recorded. The displacement mechanisms in imbibition are mainly snap-off, In type imbibition and piston-type motion. The experimental fluid displacement images and associated capillary pressure were then used to verify the displacement formulas. This experimental study shows that, when snap-off occurred, the experimental capillary pressures were close to the Lenormand's estimation of critical capillary pressures where enough surrounding area of the throat was saturated. When I1 and I2 type imbibitions occurred, the experimental capillary pressures were also close to the Lenormand's estimation of critical capillary pressures where enough connecting throats were saturated. The In type imbibition and its associated piston-type motions are the main processes to increase the wetting phase fluid saturation. For the pore-throat distribution applied in this study, snap-off can facilitate the occurrence of In type imbibition and its associated piston-type motion; therefore, snap-off is an important displacement mechanism in facilitating the increase of the wetting phase fluid saturation in the imbibition process. To summarize, this study provides valuable experimental support and suggestions for Lenormand's displacement formulas, which are the basis for many related experimental and numerical studies.