In order to make an intensive study of the failure mechanism of rectangular footings adjacent to slope, a three-dimensional and bilateral failure mode was established, which fully considered the influence of the shear strength of inside soil in the foundation and the double asymmetrical features. Moreover, a simplified construction method of the rigid-motion blocks collapse mechanism was proposed, which could not only effectively reflect the three-dimensional end effect but also avoid complex coordinate and surface integral calculation, and it is more convenient for practical engineering. Based on the failure mode, the upper limit analysis theory was then introduced, and a new analysis approach of ultimate bearing capacity of rectangular footing adjacent to slope was put forward. Meanwhile, the solving of the bearing capacity was realized by using the SQP optimization theory. Finally, the feasibility and rationality of the research approach proposed is showed through the comparison analysis with the current research as well as the ABQUS finite element results.