The formation mechanism for the body-centred regular octahedral structure of Li₇ cluster is proposed. The curve of the total energy versus the separation R between the nucleus at the centre and nuclei at the apexes for this structure of Li₇ has been calculated by using the method of Gou's modified arrangement channel quantum mechanics (MACQM). The result shows that the curve has a minimal energy of -52.16973 a.u. at R=5.06a₀. When R approaches infinity, the total energy of seven lithium atoms has the value of -51.99621 a.u. So the binding energy of Li₇ with respect to seven lithium atoms is 0.17352 a.u. Therefore the binding energy per atom for Li₇ is 0.02479 a.u. or 0.674 eV, which is greater than the binding energy per atom of 0.453 eV for Li₂, the binding energy per atom of 0.494 eV for Li₃ and the binding energy per atom of 0.632 eV for Li₅ calculated previously by us. This means that the Li₇ cluster may be formed stably in a body-centred regular octahedral structure with a greater binding energy.