Due to the difference in their longitudinal sections, foundation piles have significantly different shaft frictions and tip resistances under the vertical loading condition. Although the piles with a variable longitudinal section have extensively used in engineering, little research effort is made to investigating the failure modes of the surrounding soil or the end bearing soil under ultimate load. Based on transparent soil material and particle image velocimetry (PIV) technique, comparative model tests on the bearing capacities and the failure modes of belled wedge pile, tapered pile and uniform-section pile with an equivalent volume are carried out. The load-settlement curves of the pile top and the displacement field of the surrounding soil under different loads are measured; the failure modes under ultimate loading and the bearing capacities influenced by pile length are analyzed in details. The results show that the ultimate bearing capacity of a belled wedge pile is about 3.5 times that of the conventional tapered pile, and is approximately about 2.5 times that of the uniform-section pile under this model test condition; and the failure modes of soils around the pile tips of the variable longitudinal section piles are similar.